SUMMARY

This summary highlights selected information from this joint proxy statement/prospectus. It may not contain all of the information that is important to you. You are urged to read carefully the entire joint proxy statement/prospectus and the other documents attached to or referred to in this joint proxy statement/prospectus in order to fully understand the merger agreement and the proposed mergers. See “Where You Can Find More Information” beginning on page 229 of this joint proxy statement/prospectus. Each item in this summary refers to the page of this joint proxy statement/prospectus on which that subject is discussed in more detail.

The Companies (See Page 73)

Aetna Inc.

Aetna was incorporated in the Commonwealth of Pennsylvania in 1982. Aetna, together with its subsidiaries, is one of the nation’s leading diversified health care benefits companies, serving an estimated 46.7 million people as of June 30, 2015, with information and resources to help them in consultation with their health care professionals make better informed decisions about their health care. Aetna offers a broad range of traditional, voluntary and consumer-directed health insurance products and related services, including medical, pharmacy, dental, behavioral health, group life and disability plans, medical management capabilities, Medicaid health care management services, Medicare Advantage and Medicare supplement plans, workers’ compensation administrative services and health information technology products and services, such as Accountable Care Solutions. Aetna’s customers include employer groups, individuals, college students, part-time and hourly workers, health plans, health care providers, governmental units, government-sponsored plans, labor groups and expatriates.

The principal trading market for Aetna common shares (NYSE: AET) is the NYSE. The principal executive offices of Aetna are located at 151 Farmington Avenue, Hartford, CT 06156; its telephone number is (860) 273-0123; and its website is www.aetna.com. Information on Aetna’s Internet website is not incorporated by reference into or otherwise part of this joint proxy statement/prospectus.

This joint proxy statement/prospectus incorporates important business and financial information about Aetna from other documents that are not included in or delivered with this joint proxy statement/prospectus. For a list of the documents that are incorporated by reference, see “Where You Can Find More Information” beginning on page 229 of this joint proxy statement/prospectus.

Humana Inc.

Humana was incorporated in the State of Delaware in 1964. Headquartered in Louisville, Kentucky, Humana is a leading health and well-being company focused on making it easy for people to achieve their best health with clinical excellence through coordinated care. Humana’s strategy integrates care delivery, the member experience, and clinical and consumer insights to encourage engagement, behavior change, proactive clinical outreach and wellness for the millions of people it serves across the country. As of June 30, 2015, Humana had approximately 14.2 million members in its medical benefit plans, including 2.7 million individual Medicare Advantage members, as well as approximately 7.4 million members in its specialty products. During 2014, 73% of Humana’s total premiums and services revenue were derived from contracts with the federal government, including 15% derived from its individual Medicare Advantage contracts in Florida with the Centers for Medicare & Medicaid Services, which is referred to in this joint proxy statement/prospectus as CMS.

The principal trading market for Humana common stock (NYSE: HUM) is the NYSE. The principal executive offices of Humana are located at 500 West Main Street, Louisville, Kentucky 40202; its telephone

number is (502) 580-1000; and its website is www.humana.com. Information on Humana’s Internet website is not incorporated by reference into or otherwise part of this joint proxy statement/prospectus.

This joint proxy statement/prospectus incorporates important business and financial information about Humana from other documents that are not included in or delivered with this joint proxy statement/prospectus. For a list of the documents that are incorporated by reference, see “Where You Can Find More Information” beginning on page 229 of this joint proxy statement/prospectus.

Echo Merger Sub, Inc.

Merger Sub 1 was incorporated in the State of Delaware on June 26, 2015, and is a wholly owned subsidiary of Aetna. Merger Sub 1 was formed solely for the purpose of completing the mergers. Merger Sub 1 has not carried on any activities to date, except for activities incidental to its formation and activities undertaken in connection with the mergers.

The principal executive offices of Merger Sub 1 are located at 151 Farmington Avenue, Hartford, CT 06156; and its telephone number is (860) 273-0123.

Echo Merger Sub, LLC

Merger Sub 2 was formed in the State of Delaware on June 26, 2015, and is a wholly owned subsidiary of Aetna. Merger Sub 2 was formed solely for the purpose of completing the subsequent merger. Merger Sub 2 has not carried on any activities to date, except for activities incidental to its formation and activities undertaken in connection with the mergers.

The principal executive offices of Merger Sub 2 are located at 151 Farmington Avenue, Hartford, CT 06156; and its telephone number is (860) 273-0123.

The Mergers (See Page 89)

Aetna, Merger Subs and Humana have entered into the merger agreement. Subject to the terms and conditions of the merger agreement and in accordance with applicable law, in the merger, Merger Sub 1 will be merged with and into Humana, with Humana continuing as the surviving corporation and a wholly owned subsidiary of Aetna, and in the subsequent merger, Humana will be merged with and into Merger Sub 2, with Merger Sub 2, which will be re-named “Humana LLC”, continuing as the surviving company and a wholly owned subsidiary of Aetna. Upon completion of the merger, Humana’s common stock will no longer be publicly traded.

A copy of the merger agreement is attached as Annex A to this joint proxy statement/prospectus. You should read the merger agreement carefully because it is the legal document that governs the mergers.

Special Meeting of Stockholders of Humana (See Page 82)

Meeting. The Humana special meeting will be held on October 19, 2015, at the offices of Fried Frank, Harris, Shriver & Jacobson LLP on the 36^th floor of 375 Park Avenue, New York, New York 10152, at 3:30 p.m., Eastern Time. At the Humana special meeting, Humana stockholders will be asked to consider and vote on the following proposals:

Record Date. Humana’s board of directors has fixed the close of business on September 16, 2015, as the record date for determination of the stockholders entitled to vote at the Humana special meeting or any adjournment or postponement thereof. Only Humana stockholders of record at the record date are entitled to receive notice of, and to vote at, the Humana special meeting or any adjournment or postponement of the Humana special meeting. As of the close of business on August 25, 2015, the most recent practicable date for which such information was available, there were 148,214,812 shares of Humana common stock outstanding. Each holder of Humana common stock is entitled to one vote for each share of Humana common stock owned at the record date. The number of shares of Humana common stock outstanding as of the record date is not expected to be meaningfully different from the number as of August 25, 2015.

Quorum. The presence at the Humana special meeting, in person or by proxy, of the holders of record of a majority of the shares of Humana common stock issued and outstanding at the record date (the close of business on September 16, 2015) and entitled to vote will be necessary and sufficient to constitute a quorum at the Humana special meeting. Abstentions will be deemed present at the Humana special meeting for the purpose of determining the presence of a quorum. Shares of Humana common stock held in “street name” with respect to which the beneficial owner fails to give voting instructions to the broker, bank or other nominee holder of record on any of the proposals to be voted on at the Humana special meeting, and shares of Humana common stock with respect to which the beneficial owner otherwise fails to vote, will not be deemed present at the Humana special meeting for the purpose of determining the presence of a quorum. There must be a quorum for business to be conducted at the Humana special meeting. Failure of a quorum to be present at the Humana special meeting will necessitate an adjournment or postponement thereof and will subject Humana to additional expense.

Required Vote. Pursuant to Humana’s certificate of incorporation, which is referred to in this joint proxy statement/prospectus as Humana’s charter, to adopt the merger agreement, the affirmative vote of holders of at least three-fourths of the shares of Humana common stock outstanding and entitled to vote thereon is required. Aetna may be deemed to be a “related company” of Humana under Article Eleventh of Humana’s charter because one or more institutional stockholders of Humana who appear to beneficially own more than 5% of the outstanding shares of Humana common stock also appear to beneficially own 5% or more of the outstanding Aetna common shares. Humana cannot complete the merger and the merger consideration will not be paid unless its stockholders adopt the merger agreement by a vote “FOR” the proposal by holders of at least three-fourths of the outstanding shares of Humana common stock entitled to vote on the proposal. Because adoption of the merger agreement requires the affirmative vote of at least three-fourths of the outstanding shares of Humana common stock entitled to vote thereon, a Humana stockholder’s abstention from voting, the failure of a Humana stockholder who holds his or her shares in “street name” through a broker, bank or other nominee holder of record to give voting instructions to that broker, bank or other nominee holder of record or a Humana stockholder’s other failure to vote will have the same effect as a vote “AGAINST” the proposal to adopt the merger agreement.

To approve (i) the adjournment from time to time of the Humana special meeting if necessary to solicit additional proxies if there are not sufficient votes to adopt the merger agreement at the time of the Humana special meeting or any adjournment or postponement thereof (assuming a quorum is present) and (ii) the non-binding advisory proposal to approve compensation that will or may be paid by Humana to its named executive officers in connection with the merger, the affirmative vote of holders of a majority of the votes cast affirmatively or negatively on such proposal is required. Accordingly, a Humana stockholder’s abstention from voting, the failure of a Humana stockholder who holds his or her shares in “street name” through a broker, bank or other nominee holder of record to give voting instructions to that broker, bank or other nominee holder of record or a Humana stockholder’s other failure to vote will have no effect on the outcome of any vote to adjourn

the special meeting (assuming a quorum is present) or any vote to approve, on an advisory (non-binding) basis, compensation that will or may be paid or provided by Humana to its named executive officers in connection with the merger.

Stock Ownership of and Voting by Humana Directors and Executive Officers. At the close of business on August 25, 2015, the most recent practicable date for which such information was available, Humana’s directors and executive officers and their affiliates beneficially owned and had the right to vote in the aggregate 351,175 shares of Humana common stock, which represents 0.24% of the shares of Humana common stock entitled to vote as of that date. The number of shares of Humana common stock which Humana’s directors and executive officers and their affiliates will beneficially own as of the record date (the close of business on September 16, 2015) is not expected to be meaningfully different from the number as of August 25, 2015.

It is expected that Humana’s directors and executive officers and their affiliates will vote their shares “FOR” the adoption of the merger agreement, “FOR” the proposal to adjourn from time to time the Humana special meeting if necessary to solicit additional proxies if there are not sufficient votes to adopt the merger agreement at the time of the Humana special meeting or any adjournment or postponement thereof and “FOR” the non-binding advisory proposal to approve compensation that will or may be paid or provided by Humana to its named executive officers, although none of them has entered into any agreement requiring them to do so.

Special Meeting of Shareholders of Aetna (See Page 75)

Meeting. The Aetna special meeting will be held on October 19, 2015, at the Hilton Garden Inn, located at 85 Glastonbury Blvd., Glastonbury, Connecticut 06033, at 1:30 p.m., Eastern Time. At the Aetna special meeting, Aetna shareholders will be asked to consider and vote on the following proposals:

Record Date. Aetna’s board of directors has fixed the close of business on September 16, 2015, as the record date for determination of the shareholders entitled to vote at the Aetna special meeting or any adjournment or postponement thereof. Only Aetna shareholders of record at the record date are entitled to receive notice of, and to vote at, the Aetna special meeting or any adjournment or postponement of the Aetna special meeting. As of the close of business on August 25, 2015, the most recent practicable date for which such information was available, there were 348,688,145 Aetna common shares outstanding. Each holder of Aetna common shares is entitled to one vote for each Aetna common share owned at the record date. The number of Aetna common shares outstanding as of the record date is not expected to be meaningfully different from the number as of August 25, 2015.

Quorum. The presence at the Aetna special meeting, in person or by proxy, of the holders of a majority of the outstanding Aetna common shares at the record date (the close of business on September 16, 2015) will constitute a quorum. Abstentions will be deemed present at the Aetna special meeting for the purpose of determining the presence of a quorum. Aetna common shares held in “street name” with respect to which the beneficial owner fails to give voting instructions to the broker, bank or other nominee holder of record, and Aetna common shares with respect to which the beneficial owner otherwise fails to vote, will not be deemed present at the Aetna special meeting for the purpose of determining the presence of a quorum. There must be a quorum for the vote on the share issuance to be taken at the Aetna special meeting. Failure of a quorum to be present at the Aetna special meeting will necessitate an adjournment of the meeting and will subject Aetna to additional expense. Pursuant to Pennsylvania law, if the Aetna special meeting is adjourned for one or more periods aggregating at least 15 days due to the absence of a quorum, Aetna shareholders who are entitled to vote

and who attend (including by proxy) the adjourned meeting, even though they do not constitute a quorum as described above, will constitute a quorum for the purpose of acting on the stock issuance.

Required Vote. Assuming a quorum is present, to approve the stock issuance, the affirmative vote of a majority of the votes cast at the Aetna special meeting by holders of Aetna common shares is required. Aetna cannot complete the merger unless its shareholders approve the stock issuance. Under the current rules and interpretive guidance of the NYSE, “votes cast” on the stock issuance consist of votes “for” or “against” as well as abstentions. As a result, an Aetna shareholder’s abstention from voting on the stock issuance will have the same effect as a vote “AGAINST” the proposal. The failure of an Aetna shareholder who holds his or her shares in “street name” through a broker, bank or other nominee holder of record to give voting instructions to that broker, bank or other nominee holder of record or an Aetna shareholder’s other failure to vote will have no effect on the outcome of any vote to approve the stock issuance because these failures to vote are not considered “votes cast.”

To approve the adjournment of the Aetna special meeting if necessary to solicit additional proxies if there are not sufficient votes to approve the stock issuance at the time of the Aetna special meeting, whether or not a quorum, as defined under Pennsylvania law, is present, the affirmative vote of a majority of the votes cast at the Aetna special meeting by Aetna shareholders is required. For purposes of the adjournment proposal, “votes cast” means votes “for” or “against” the proposal. As a result, an Aetna shareholder’s abstention from voting, the failure of an Aetna shareholder who holds his or her shares in “street name” through a broker, bank or other nominee holder of record to give voting instructions to that broker, bank or other nominee holder of record or an Aetna shareholder’s other failure to vote will have no effect on the outcome of any vote to adjourn the Aetna special meeting.

Share Ownership of and Voting by Aetna Directors and Executive Officers. At the close of business on August 25, 2015, the most recent practicable date for which such information was available, Aetna’s directors and executive officers and their affiliates beneficially owned and had the right to vote in the aggregate 1,207,079 Aetna common shares, which represents 0.35% of the Aetna common shares entitled to vote as of that date. The number of Aetna common shares which Aetna’s directors and executive officers and their affiliates will beneficially own as of the record date (the close of business on September 16, 2015) is not expected to be meaningfully different from the number as of August 25, 2015.

It is expected that Aetna’s directors and executive officers and their affiliates will vote their shares “FOR” the stock issuance and “FOR” the adjournment of the Aetna special meeting if necessary to solicit additional proxies if there are not sufficient votes to approve the stock issuance at the time of the Aetna special meeting, although none of them has entered into any agreement requiring them to do so.

What Humana Stockholders Will Receive in the Merger (See Page 158)

If the merger is completed, Humana stockholders will be entitled to receive, in exchange for each share of Humana common stock that they own immediately prior to the merger, $125.00 in cash, without interest, and 0.8375 of an Aetna common share, together with cash payable in lieu of any fractional shares as described below.

Aetna will not issue any fractional shares in the merger. Instead, the total number of Aetna common shares that each Humana stockholder will receive in the merger will be rounded down to the nearest whole number, and each Humana stockholder will receive cash, without interest, for any fractional Aetna common share that he or she would otherwise receive in the merger. The amount of cash for fractional shares will be calculated by multiplying the fraction of an Aetna common share that the Humana stockholder would otherwise be entitled to receive in the merger by the Aetna closing price.

Example: If you own 100 shares of Humana common stock at the time the merger is completed, you will be entitled to receive $12,500.00 in cash, without interest, and 83 Aetna common shares. In addition, you will be entitled to receive an amount of cash equal to 0.75 of an Aetna common share multiplied by the Aetna closing price.

The exchange ratio is fixed, which means that it will not change between now and the date of the merger, regardless of whether the market price of either Aetna common shares or Humana common stock changes. Therefore, the value of the stock portion of the merger consideration will depend on the market price of Aetna common shares at the time Humana stockholders receive Aetna common shares in the merger. Based on the closing price of an Aetna common share on the NYSE on July 2, 2015, the last trading day before the public announcement of the merger agreement, the merger consideration represented approximately $230.11 in value for each share of Humana common stock. Based on the closing price of an Aetna common share on the NYSE on August 27, 2015, the most recent trading day prior to the date of this joint proxy statement/prospectus for which this information was available, the merger consideration represented approximately $223.64 in value for each share of Humana common stock. The market price of Aetna common shares has fluctuated since the date of the announcement of the merger agreement and will continue to fluctuate from the date of this joint proxy statement/prospectus to the date of the Humana special meeting and the date the merger is completed and thereafter. The market price of Aetna common shares when received by Humana stockholders after the merger is completed could be greater than, less than or the same as the market price of Aetna common shares on the date of this joint proxy statement/prospectus or at the time of the Humana special meeting or any adjournment or postponement thereof.

Treatment of Humana Equity Awards (See Page 160)

As of the completion of the merger, each vested option to purchase shares of Humana common stock, which are referred to in this joint proxy statement/prospectus as Humana stock options (including those Humana stock options that vest by their terms as of the completion of the merger), will be cancelled and converted into the right to receive a cash amount equal to, for each share of Humana common stock underlying the Humana stock option: the excess, if any, of (i) the equity award cash consideration over (ii) the applicable per share exercise price of the Humana stock option. Each outstanding vested Humana stock option (including those Humana stock options that vest by their terms as of the completion of the merger) with a per-share exercise price greater than or equal to the equity award cash consideration, will be cancelled for no consideration.

Each Humana stock option that is not vested as of the completion of the merger or that is granted after the date of the merger agreement (to the extent permitted under the merger agreement), at the completion of the merger, will be assumed by Aetna and will become an option to purchase Aetna common shares on the same terms and conditions, except that (i) the number of Aetna common shares subject to the assumed stock option will equal (A) the number of shares of Humana common stock that were subject to such Humana stock option immediately prior to the completion of the merger, multiplied by (B) the sum of (1) the exchange ratio, plus (2) the quotient of (x) $125.00, divided by (y) the Aetna closing price (the sum of the amounts in clauses (1) and (2) is referred to in this joint proxy statement/prospectus as the equity award exchange ratio) and (ii) the per-share exercise price will equal the exercise price per share of the Humana stock option immediately prior to the completion of the merger, divided by the equity award exchange ratio.

Immediately prior to the completion of the merger, each outstanding Humana RSU and Humana PSU that provides for accelerated vesting upon the completion of the transactions contemplated by the merger agreement will vest and will be converted into the right to receive, with respect to each share of Humana common stock underlying the Humana RSU or Humana PSU, the merger consideration, plus a cash amount equal to the accrued but unpaid dividend equivalent rights as of the completion of the merger relating to such Humana RSU or Humana PSU. Humana PSUs will be earned assuming the achievement of the maximum level of performance to the extent required by their terms.

Each Humana RSU or Humana PSU that is not converted into a right to receive the merger consideration and the other amounts referred to above, or that is granted after the date of the merger agreement (to the extent permitted under the merger agreement), will be assumed by Aetna and will be converted into a restricted unit award or performance-based unit award, as applicable, that settles in, with respect to each share of Humana common stock underlying the Humana RSU or Humana PSU (with the performance of the Humana PSU to be determined based on the agreement relating to the Humana PSU), the merger consideration, plus a cash amount equal to the accrued but unpaid dividend equivalent rights relating to the Humana RSU or Humana PSU.

Recommendations of the Humana Board of Directors (See Page 82)

Humana’s board of directors unanimously determined that the merger agreement, the merger, and the other transactions contemplated by the merger agreement are advisable and fair to and in the best interests of Humana stockholders. Humana’s board of directors unanimously recommends that Humana stockholders vote “FOR” the proposal to adopt of the merger agreement. For the factors considered by Humana’s board of directors in reaching this decision, see “Humana Proposal I: Adoption of the Merger Agreement and Aetna Proposal I: Approval of the Stock Issuance—Humana’s Reasons for the Mergers; Recommendation of the Humana Board of Directors that Humana Stockholders Adopt the Merger Agreement” beginning on page 104 of this joint proxy statement/prospectus.

Humana’s board of directors unanimously recommends that Humana stockholders vote “FOR” the proposal to adjourn from time to time the Humana special meeting if necessary to solicit additional proxies if there are not sufficient votes to adopt the merger agreement at the time of the Humana special meeting or any adjournment or postponement thereof. See “Humana Proposal II: Adjournment of the Humana Special Meeting” beginning on page 197 of this joint proxy statement/prospectus.

In addition, Humana’s board of directors unanimously recommends that Humana stockholders vote “FOR” the proposal to approve, on an advisory (non-binding) basis, compensation that will or may be paid or provided by Humana to its named executive officers in connection with the merger. See “Humana Proposal III: Advisory Vote On Merger-Related Executive Compensation Arrangements” beginning on page 198 of this joint proxy statement/prospectus.

Recommendations of the Aetna Board of Directors (See Page 75)

Aetna’s board of directors unanimously determined that the merger agreement, the merger, and the other transactions contemplated by the merger agreement, including the stock issuance, are advisable and fair to and in the best interests of Aetna shareholders. Aetna’s board of directors unanimously recommends that Aetna shareholders vote “FOR” the stock issuance. For the factors considered by Aetna’s board of directors in reaching this decision, see “Humana Proposal I: Adoption of the Merger Agreement and Aetna Proposal I: Approval of the Stock Issuance—Aetna’s Reasons for the Merger; Recommendation of the Aetna Board of Directors that Aetna Shareholders Approve the Stock Issuance” beginning on page 109 of this joint proxy statement/prospectus.

Aetna’s board of directors unanimously recommends that Aetna shareholders vote “FOR” the adjournment of the Aetna special meeting if necessary to solicit additional proxies if there are not sufficient votes to approve the stock issuance at the time of the Aetna special meeting. See “Aetna Proposal II: Adjournment of the Aetna Special Meeting” beginning on page 199 of this joint proxy statement/prospectus.

Opinion of Humana’s Financial Advisor (See Page 113)

Goldman, Sachs & Co., which is referred to in this joint proxy statement/prospectus as Goldman Sachs, delivered its opinion to Humana’s board of directors that, as of July 2, 2015, and based upon and subject to the factors and assumptions set forth therein, the merger consideration to be paid to holders (other than Aetna and its

affiliates) of shares of Humana’s common stock pursuant to the merger agreement was fair from a financial point of view to such holders.

The full text of the written opinion of Goldman Sachs, dated July 2, 2015, which sets forth assumptions made, procedures followed, matters considered, qualifications and limitations on the review undertaken in connection with the opinion, is attached as Annex C to this joint proxy statement/prospectus. Goldman Sachs provided its opinion for the information and assistance of Humana’s board of directors in connection with its consideration of the mergers. The Goldman Sachs opinion does not constitute a recommendation as to how any holder of Humana’s common stock should vote with respect to the mergers or any other matter.

For further information, see the section of this joint proxy statement/prospectus entitled “Humana Proposal I: Adoption of the Merger Agreement and Aetna Proposal I: Approval of the Stock Issuance—Opinion of Humana’s Financial Advisor” beginning on page 113 of this joint proxy statement/prospectus and Annex C.

Opinions of Aetna’s Financial Advisors (See Page 120)

Opinion of Citigroup Global Markets Inc.

On July 2, 2015, Citigroup Global Markets Inc., which is referred to in this joint proxy statement/prospectus as Citi, rendered its oral opinion to Aetna’s board of directors, which was confirmed by delivery of a written opinion dated July 2, 2015, to the effect that, as of such date and based on and subject to the assumptions made, procedures followed, matters considered and limitations and qualifications set forth in the written opinion, each as described in greater detail in the section titled “Humana Proposal I: Adoption of the Merger Agreement and Aetna Proposal I: Approval of the Stock Issuance—Opinions of Aetna’s Financial Advisors—Opinion of Citigroup Global Markets Inc.” beginning on page 120 of this joint proxy statement/prospectus, the merger consideration to be paid by Aetna in the merger was fair, from a financial point of view, to Aetna. Citi’s opinion, the issuance of which was authorized by Citi’s fairness opinion committee, was provided to Aetna’s board of directors in connection with its evaluation of the mergers and was limited to the fairness, from a financial point of view, as of the date of the opinion, to Aetna of the consideration to be paid by Aetna in the mergers. Citi’s opinion does not address any other aspects or implications of the mergers and does not constitute a recommendation to any shareholder as to how such shareholder should vote or act on any matters relating to the mergers. The summary of Citi’s opinion is qualified in its entirety by reference to the full text of the opinion. We encourage you to read the full text of Citi’s written opinion, which is attached to this joint proxy statement/prospectus as Annex D and sets forth, among other things, the assumptions made, procedures followed, matters considered and limitations and qualifications on the scope of review undertaken.

Opinion of Lazard Frères & Co. LLC

On July 2, 2015, Lazard Frères & Co. LLC, which is referred to in this joint proxy statement/prospectus as Lazard, rendered its oral opinion to Aetna’s board of directors, subsequently confirmed in writing, that, as of such date, and based upon and subject to the assumptions, procedures, factors, qualifications and limitations set forth therein, the merger consideration to be paid by Aetna in the merger was fair, from a financial point of view, to Aetna.

The full text of Lazard’s written opinion, dated July 2, 2015, which sets forth, among other things, the assumptions made, procedures followed, factors considered, and qualifications and limitations on the review undertaken by Lazard in connection with its opinion is attached to this joint proxy statement/prospectus as Annex E and is incorporated into this joint proxy statement/prospectus by reference. We encourage you to read Lazard’s opinion, and the section titled “Humana Proposal I: Adoption of the Merger Agreement and Aetna Proposal I: Approval of the Stock Issuance—Opinions of Aetna’s Financial Advisors—Opinion of Lazard Frères & Co. LLC” beginning on page 129 of this joint proxy statement/prospectus, carefully and in their entirety.

Lazard’s opinion was directed to and for the benefit of Aetna’s board of directors (in their capacity as such) for the information and assistance of the Aetna board of directors in connection with its evaluation of the mergers and only addressed the fairness, from a financial point of view, to Aetna of the merger consideration to be paid by Aetna in the merger as of the date of Lazard’s opinion. Lazard’s opinion did not address any other aspect of the mergers and was not intended to and does not constitute a recommendation to any shareholder as to how such shareholder should vote or act with respect to the merger or any matter relating thereto.

Ownership of Aetna Common Shares After the Mergers (See Page 89)

Based on the number of shares of Humana common stock (including the number of shares underlying Humana RSUs and PSUs) outstanding as of August 25, 2015, the most recent practicable date for which such information was available, and the treatment of Humana shares, RSUs and PSUs in the merger, Aetna expects to issue approximately 127.0 million Aetna common shares to Humana stockholders upon completion of the merger and reserve for issuance additional Aetna common shares in connection with the assumption, exercise and settlement of Humana options, RSUs and PSUs that are not vested as of the completion of the merger or that are granted after the date of the merger agreement (to the extent permitted by the merger agreement). The actual number of Aetna common shares to be issued and reserved for issuance upon completion of the merger will be determined at completion of the merger based on the exchange ratio and the number of shares of Humana common stock (including the number of shares underlying Humana RSUs and PSUs) outstanding at that time. Based on the number of shares of Humana common stock (including the number of shares underlying Humana options, Humana RSUs and PSUs) outstanding as of August 25, 2015, and the number of Aetna common shares outstanding as of August 25, 2015, the most recent practicable date for which such information was available, it is expected that, immediately after completion of the mergers, former Humana stockholders will own approximately 27% of the outstanding Aetna common shares.

Governance Following Completion of the Mergers (See Page 161)

Following completion of the mergers, Aetna will maintain the corporate headquarters of the combined company’s Medicare, Medicaid and TRICARE businesses in Louisville, Kentucky, and Humana LLC will maintain a significant corporate presence in Louisville, Kentucky.

As of completion of the mergers, the size of the board of directors of Aetna will be expanded to include four members of the board of directors of Humana who are independent with respect to Aetna and jointly designated by Humana and Aetna. As of the date of this joint proxy statement/prospectus, Aetna and Humana have not made a determination as to which four members of the board of directors of Humana will be designated to Aetna’s board of directors. Mark T. Bertolini will continue to serve as Chairman and Chief Executive Officer of Aetna following the completion of the mergers. See “The Merger Agreement—Governance Following Completion of the Mergers” beginning on page 161 for further information.

Interests of Humana’s Directors and Executive Officers in the Merger (See Page 188)

In considering the recommendation of Humana’s board of directors to adopt the merger agreement, Humana stockholders should be aware that Humana’s directors and executive officers have interests in the merger that may be different from, or in addition to, the interests of Humana stockholders generally. Humana’s board of directors was aware of these interests and considered them, among other matters, in evaluating and negotiating the merger agreement, in reaching its decision to approve the merger agreement and in recommending to Humana stockholders that the merger agreement be adopted.

These interests include the following:

These interests are described in further detail, including more information on the assumptions used in calculating the estimated amounts set forth above, under “Interests of Humana’s Directors and Executive Officers in the Merger” and “The Merger Agreement—Indemnification and Insurance” beginning on pages 188 and 180, respectively, of this joint proxy statement/prospectus.

Listing of Aetna Common Shares and Delisting and Deregistration of Humana Common Stock (See Page 152)

Aetna will apply to have the Aetna common shares to be issued in the merger approved for listing on the NYSE, where Aetna common shares are currently traded. If the merger is completed, shares of Humana common stock will no longer be listed on the NYSE and will be deregistered under the Securities Exchange Act of 1934, as amended, which is referred to in this joint proxy statement/prospectus as the Exchange Act.

Appraisal Rights Available to Humana Stockholders (See Page 145)

Under Section 262 of the DGCL, if the merger is completed, holders of Humana common stock who do not vote in favor of the adoption of the merger agreement and who otherwise properly comply with the procedures for exercising appraisal rights under Section 262 of the DGCL will be entitled, in lieu of receiving the merger consideration, to obtain payment in cash of the fair value of their shares of Humana common stock as determined by the Delaware Court of Chancery. The judicially determined fair value could be more than, the same as, or less than the value of the merger consideration. Section 262 of the DGCL is included as Annex B to this joint proxy statement/prospectus. Holders of shares of Humana common stock are encouraged to read Section 262 of the DGCL carefully and in its entirety. Moreover, due to the complexity of the procedures for exercising appraisal rights, Humana stockholders who are considering exercising appraisal rights are encouraged to seek the advice of legal counsel. Failure to comply properly with the provisions of Section 262 of the DGCL will result in loss of appraisal rights and receipt of the merger consideration.

Appraisal or Dissenters’ Rights Not Available to Aetna Shareholders (See Page 149)

Under Pennsylvania law, Aetna shareholders will not be entitled to appraisal or dissenters’ rights in connection with the mergers.

Completion of the Mergers Is Subject to Certain Conditions (See Page 161)

As more fully described in this joint proxy statement/prospectus and in the merger agreement, the obligation of each of Aetna and Merger Subs, on the one hand, and Humana, on the other hand, to complete the mergers is subject to the satisfaction (or, to the extent permitted by applicable law, waiver) of a number of conditions, including the following:

In addition, Aetna’s obligations to complete the mergers are subject to the satisfaction (or, to the extent permitted by applicable law, waiver) of certain additional conditions, including the following:

In addition, the obligation of Humana to complete the mergers is subject to the satisfaction (or, to the extent permitted by applicable law, waiver) of certain additional conditions, including (i) expiration or early termination of the waiting period relating to the merger under the HSR Act and (ii) the required governmental authorizations having been made or obtained.

Aetna and Humana cannot be certain when, or if, the conditions to the mergers will be satisfied or waived, or that the mergers will be completed.

The Mergers May Not Be Completed Without All Required Regulatory Approvals (See Page 144)

Completion of the mergers is conditioned upon the expiration or early termination of the waiting period relating to the merger under the HSR Act and the required governmental authorizations having been made or

obtained and being in full force and effect, and in the case of Aetna, without the imposition of any term or condition that would have or would reasonably be expected to have, individually or in the aggregate, a regulatory material adverse effect on Aetna or Humana.

Under the HSR Act, certain transactions, including the merger, may not be completed unless certain waiting period requirements have expired or been terminated. The HSR Act provides that each party must file a pre-merger notification with the Federal Trade Commission, which is referred to in this joint proxy statement/prospectus as the FTC, and the Antitrust Division of the U.S. Department of Justice, which is referred to in this joint proxy statement/prospectus as the DOJ. A transaction notifiable under the HSR Act may not be completed until the expiration of a 30-calendar-day waiting period following the parties’ filings of their respective HSR Act notification forms or the early termination of that waiting period. If the DOJ or the FTC issues a Request for Additional Information and Documentary Material prior to the expiration of the initial waiting period, the parties must observe a second 30-calendar-day waiting period, which would begin to run only after both parties have substantially complied with the request for additional information, unless the waiting period is terminated earlier. Each of Aetna and Humana filed its required HSR Act notification and report with respect to the mergers on July 16, 2015. On August 19, 2015, Aetna re-filed its premerger notification under the HSR Act with the DOJ and FTC, which initiated a new waiting period that will expire on September 18, 2015, unless it is extended by a request for additional information or terminated earlier.

Pursuant to federal healthcare laws and regulations and the insurance laws and regulations and, in some instances, the healthcare laws and regulations of certain states, and pursuant to certain licenses and contracts of certain of Humana’s subsidiaries, applicable federal and state regulatory authorities must approve, or be notified of, Aetna’s acquisition of control of Humana’s health maintenance organizations, insurance companies, pharmacy businesses and other regulated businesses or entities. To obtain these approvals and provide such notices, Aetna, or the applicable Aetna subsidiary, and in some instances Humana, or the applicable Humana regulated entity, as the case may be, has filed or will file acquisition of control and material modification or similar statements, notices or applications, as required by federal healthcare law or regulation and the insurance and healthcare laws and regulations of each applicable state or the Humana regulated entities’ licenses and contracts.

Neither Aetna nor Humana is aware of any material governmental approvals or actions that are required for completion of the mergers other than those described above. It is presently contemplated that if any such additional material governmental approvals or actions are required, those approvals or actions will be sought.

Aetna and Humana have agreed to use their respective reasonable best efforts to obtain all regulatory approvals required to complete the mergers, which reasonable best efforts include contesting any proceeding brought by a governmental authority seeking to prohibit completion of the mergers or seeking damages or to impose any terms or conditions in connection with the mergers. In using its reasonable best efforts, under the terms of the merger agreement, Aetna is required to take all actions and do all things necessary, proper or advisable to complete the mergers in connection with (i) the expiration or early termination of the waiting period relating to the merger under the HSR Act, (ii) any other antitrust law or (iii) the required governmental authorizations, except that Aetna is not required to take any action or agree to any term or condition in connection with those matters if that action, term or condition would have or would reasonably be expected to have, individually or in the aggregate, a regulatory material adverse effect on Aetna or on Humana. In addition, in connection with obtaining the regulatory approvals required to complete the mergers, (x) neither Aetna nor Humana is required to take any action or agree to any term or condition that is not conditioned upon completion of the mergers and (y) Humana is not permitted to take any action or agree to any term or condition without Aetna’s consent.

Description of Debt Financing (See Page 153)

The mergers are not subject to a financing condition. On July 30, 2015, Aetna entered into a 364-day senior unsecured bridge credit agreement, which is referred to in this joint proxy statement/prospectus as the bridge credit agreement, with a group of fifteen lenders, who are collectively referred to in this joint proxy statement/prospectus as the bridge lenders, to finance up to $13.0 billion of the cash consideration and fees and expenses payable by Aetna in connection with the mergers to the extent that Aetna has not received $13.0 billion of net cash proceeds from the issuance of Aetna’s senior notes or from certain other transactions at or prior to completion of the mergers. The bridge lenders’ obligation to fund the bridge loan is subject to several conditions as set forth in the bridge credit agreement, including, among others, completion of the mergers, the non-occurrence of a material adverse effect on Humana, the accuracy of certain representations and warranties related to both Aetna and Humana, the absence of certain defaults by Aetna, Aetna’s satisfaction of a maximum ratio of consolidated total indebtedness to adjusted consolidated capitalization, Aetna’s and Humana’s delivery of certain financial statements, the termination of Humana’s existing credit agreement dated as of July 9, 2013, which is referred to in this joint proxy statement/prospectus as Humana’s existing credit agreement, Aetna having used commercially reasonable efforts to cause the Aetna senior notes to be issued and other conditions to completion.

On July 30, 2015, Aetna entered into a senior unsecured three year term loan credit agreement, which is referred to in this joint proxy statement/prospectus as the term loan agreement, with a group of seventeen lenders, who are collectively referred to in this joint proxy statement/prospectus as the term lenders, to finance up to $3.2 billion of the cash consideration and fees and expenses payable by Aetna in connection with the mergers. The term lenders’ obligation to fund the term loan is subject to several conditions as set forth in the term loan agreement, including, among others, completion of the mergers, the non-occurrence of a material adverse effect on Humana, the accuracy of certain representations and warranties related to both Aetna and Humana, the absence of certain defaults by Aetna, Aetna’s satisfaction of a maximum ratio of consolidated total indebtedness to adjusted consolidated capitalization, Aetna’s and Humana’s delivery of certain financial statements, the termination of Humana’s existing credit agreement and other conditions to completion.

On July 30, 2015, Aetna and its current group of nineteen lenders entered into the third amendment to Aetna’s existing five-year revolving credit agreement to permit Aetna to increase the commitments available under that agreement from $2.0 billion to $3.0 billion upon Aetna’s request. In this joint proxy statement/prospectus, Aetna’s existing revolving credit agreement is referred to as the revolving credit agreement, and the lenders under the revolving credit agreement are referred to as the revolving lenders. Aetna expects to upsize its commercial paper program to $3.0 billion and use its commercial paper program, which is supported by the revolving credit agreement, to finance a portion of the cash consideration and fees and expenses payable by Aetna in connection with the mergers. Aetna also may use the revolving credit agreement to finance a portion of the cash consideration and fees and expenses payable by Aetna in connection with the mergers. The revolving lenders’ obligation to increase their commitments under the revolving credit agreement to $3.0 billion is subject to several conditions as set forth in the third amendment to the revolving credit agreement, including, among others, completion of the mergers, the termination of Humana’s existing credit agreement and other conditions to completion.

For a more complete description of Aetna’s debt financing for the merger, see “Humana Proposal I: Adoption of the Merger Agreement and Aetna Proposal I: Approval of the Stock Issuance—Description of Debt Financing” beginning on page 153 of this joint proxy statement/prospectus.

Aetna and Humana Expect the Mergers to be Completed in the Second Half of 2016 (See Page 158)

The mergers will occur three business days after the conditions to their completion have been satisfied or, to the extent permitted by applicable law, waived, unless otherwise mutually agreed by the parties. As of the date of this joint proxy statement/prospectus, Aetna and Humana expect the mergers to be completed in the second half of 2016. However, there can be no assurance as to when, or if, the mergers will occur.

No Solicitation by Humana or Aetna (See Page 172)

As more fully described in this joint proxy statement/prospectus and in the merger agreement, and subject to the exceptions described below and in the merger agreement, each of Humana and Aetna has agreed not to, among other things, (i) solicit, initiate or take any action to knowingly facilitate or knowingly encourage the submission of any acquisition proposal from any third party, (ii) enter into or participate in any discussions or negotiations with any third party that such party knows is seeking to make, or has made, an acquisition proposal, (iii) fail to make or withdraw or qualify, amend or modify in any manner adverse to the other party the recommendation of such party’s board of directors that its stockholders adopt the merger agreement, in the case of Humana, or its shareholders approve the stock issuance, in the case of Aetna, or (iv) fail to enforce or grant any waiver or release under any standstill or similar agreement.

However, at any time prior to the adoption of the merger agreement by Humana stockholders, in the case of Humana, or the approval of the stock issuance by Aetna shareholders, in the case of Aetna, subject to the terms and conditions described in the merger agreement, each of Humana or Aetna, as applicable, is permitted to:

Humana or Aetna, as applicable, is only permitted to take the actions described above if its board of directors determines that the failure to take that action would be reasonably likely to be inconsistent with its fiduciary duties under applicable law. In addition, Humana’s board of directors is not permitted to withdraw or modify its recommendation that Humana stockholders adopt the merger agreement, and Aetna’s board of directors is not permitted to withdraw or modify its recommendation that Aetna shareholders approve the stock issuance, unless, before taking that action, the applicable party notifies the other party that it intends to take that action and, if requested by the other party, negotiates in good faith with the other party for certain periods of time regarding any proposal by the other party to amend the terms of the merger agreement.

Termination of the Merger Agreement (See Page 183)

As more fully described in this joint proxy statement/prospectus and in the merger agreement, and subject to the terms and conditions set forth in the merger agreement, the merger agreement may be terminated at any time before completion of the mergers in any of the following ways:

Termination Fees (See Page 185)

As more fully described in this joint proxy statement/prospectus and in the merger agreement, and subject to the terms and conditions of the merger agreement, Humana has agreed to pay Aetna a termination fee of $1.314 billion if the merger agreement is terminated under any of the following circumstances:

As more fully described in this joint proxy statement/prospectus and in the merger agreement, and subject to the terms and conditions of the merger agreement, Aetna has agreed to pay Humana a termination fee of $1.691 billion if the merger agreement is terminated under any of the following circumstances:

In addition, as more fully described in this joint proxy statement/prospectus and in the merger agreement, and subject to the terms and conditions of the merger agreement, Aetna has agreed to pay Humana a termination fee of $1 billion if both (x) the merger agreement is terminated under any the following circumstances:

and (y) at the time of termination of the merger agreement, all of the conditions to Aetna’s and Merger Subs’ obligations to complete the mergers are satisfied other than (i) the regulatory approvals condition and (ii) the condition requiring the absence of any applicable law or order being in effect in the U.S. or any of its territories that prohibits the completion of the mergers (but only if that condition is not satisfied solely due to any applicable law or final and non-appealable order in respect of certain regulatory matters).

Except in the case of fraud, if either party receives a termination fee in accordance with the provisions of the merger agreement, the receipt of the termination fee will be the receiving party’s sole and exclusive remedy against the paying party.

See “The Merger Agreement—Termination Fee and Expenses” beginning on page 185 of this joint proxy statement/prospectus for a more complete description of the circumstances under which Humana or Aetna will be required to pay a termination fee.

Specific Performance; Remedies (See Page 186)

Under the merger agreement, each of Aetna and Humana is entitled to an injunction or injunctions to prevent breaches of the merger agreement and to specifically enforce the terms and provisions of the merger agreement.

Material U.S. Federal Income Tax Consequences (See Page 149)

Aetna and Humana intend that the mergers, taken together, will qualify as a “reorganization” within the meaning of Section 368(a) of the Code and that Aetna and Humana will each be a party to the “reorganization,” and it is a condition to the obligation of each of Aetna and Humana to complete the mergers that each shall have received an opinion from its tax counsel substantially to this effect. Accordingly, a U.S. person that is a beneficial owner of Humana common stock generally will recognize gain (but not loss) in an amount equal to the lesser of (i) the amount of cash received and (ii) the excess of the sum of the amount of cash received and the fair market value of the Aetna common shares received over the U.S. person’s adjusted tax basis in the shares of Humana common stock surrendered in the mergers.

Each Humana stockholder is urged to read the discussion in the section entitled “Humana Proposal I: Adoption of the Merger Agreement and Aetna Proposal I: Approval of the Stock Issuance—Material U.S. Federal Income Tax Consequences” beginning on page 149 of this joint proxy statement/prospectus and to consult its tax advisor to determine the particular U.S. federal, state or local or non-U.S. income or other tax consequences to it of the mergers.

Accounting Treatment (See Page 152)

The merger will be accounted for as an acquisition of a business. Aetna will record assets acquired and liabilities assumed from Humana primarily at their respective fair values at the date of completion of the mergers. Any excess of the purchase price (as described under Note 4. Estimate of Consideration Expected to be Transferred under “Aetna and Humana Unaudited Pro Forma Condensed Combined Financial Statements” beginning on page 44 of this joint proxy statement/prospectus) over the net fair value of such assets and liabilities will be recorded as goodwill.

Rights of Humana Stockholders Will Change as a Result of the Merger (See Page 208)

Humana stockholders will have different rights once they become Aetna shareholders due to differences between the organizational documents of Aetna and Humana and differences between Pennsylvania law, under which Aetna is incorporated, and Delaware law, under which Humana is incorporated. These differences are described in more detail under “Comparison of Stockholder Rights” beginning on page 208 of this joint proxy statement/prospectus.

Litigation Relating to the Mergers (See Page 152)

As of August 27, 2015, three putative class action complaints have been filed by purported Humana stockholders challenging the mergers, two in the Circuit Court of Jefferson County, Kentucky and one in the Court of Chancery of the State of Delaware. The complaints are captioned Solak v. Broussard et al., Civ. Act. No. 15CI03374 (Kentucky state court), Litwin v. Broussard et al., Civ. Act. No. 15CI04054 (Kentucky state court) and Scott v. Humana Inc. et al., C.A. No. 11323-VCL (Delaware state court). The complaints name as defendants each member of Humana’s board of directors, Aetna, Merger Subs and, in the case of the Delaware complaint, Humana. The complaints generally allege, among other things, that the individual members of Humana’s board of directors breached their fiduciary duties owed to the stockholders of Humana by entering into the merger agreement, approving the mergers, and failing to take steps to maximize the value of Humana to its stockholders, and that Aetna, Merger Subs and, in the case of the Delaware complaint, Humana aided and abetted such breaches of fiduciary duties. In addition, the complaints allege that the merger undervalues Humana, that the process leading up to the execution of the merger agreement was flawed, that the members of Humana’s board of directors improperly placed their own financial interests ahead of those of Humana’s stockholders, and that certain provisions of the merger agreement improperly favor Aetna and impede a potential alternative transaction. Among other remedies, the complaints seek equitable relief rescinding the merger agreement and enjoining the defendants from completing the mergers as well as costs and attorneys’ fees. On August 20, 2015, the parties in the Kentucky state cases filed a stipulation and proposed order with the court to consolidate those cases into a single action captioned In re Humana Inc. Shareholder Litigation, Civ. Act. No. 15CI03374. Defendants believe that the complaints are entirely without merit.

SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA OF AETNA

The following table presents selected historical consolidated financial data of Aetna. The selected historical consolidated financial data of Aetna for each of the years ended December 31, 2014, 2013 and 2012, and as of December 31, 2014 and 2013, are derived from Aetna’s audited consolidated financial statements and related notes contained in its Annual Report on Form 10-K for the year ended December 31, 2014, which is incorporated by reference into this joint proxy statement/prospectus. The selected historical consolidated financial data of Aetna for each of the years ended December 31, 2011 and 2010, and as of December 31, 2012, 2011 and 2010, have been derived from Aetna’s audited consolidated financial statements for such years, which have not been incorporated by reference into this joint proxy statement/prospectus.

The selected historical consolidated financial data of Aetna as of, and for the six months ended, June 30, 2015 and for the six months ended June 30, 2014, are derived from Aetna’s unaudited consolidated financial statements and related notes contained in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2015, which is incorporated by reference into this joint proxy statement/prospectus. The selected historical consolidated financial data of Aetna as of June 30, 2014 are derived from Aetna’s unaudited consolidated financial statements and related notes contained in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2014, which has not been incorporated by reference into this joint proxy statement/prospectus. Aetna’s management believes that Aetna’s unaudited consolidated financial statements have been prepared on a basis consistent with its audited financial statements and include all normal and recurring adjustments necessary for a fair presentation of the results for each interim period.

You should read the following selected historical consolidated financial data of Aetna in conjunction with Aetna’s audited consolidated financial statements contained in its Annual Report on Form 10-K for the year ended December 31, 2014 and unaudited consolidated financial statements and related notes contained in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2015. In particular, see the notes to Aetna’s audited consolidated financial statements for significant events affecting the comparability of results as well as material uncertainties regarding Aetna’s future financial condition and results of operations. Aetna acquired Coventry Health Care, Inc. in May 2013, which impacts the comparability of financial results for the years ended December 31, 2014 and 2013 to prior periods.

SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA OF HUMANA

The following table presents selected historical consolidated financial data of Humana. The selected historical consolidated financial data of Humana for each of the years ended December 31, 2014, 2013 and 2012, and as of December 31, 2014 and 2013, are derived from Humana’s audited consolidated financial statements and related notes contained in its Annual Report on Form 10-K for the year ended December 31, 2014, as updated by Humana’s Current Report on Form 8-K filed with the SEC on August 10, 2015 (which, including its related exhibits, updated certain information included in Humana’s Annual Report on Form 10-K for the year ended December 31, 2014, including Items 1, 6, 7 and 8, as it relates to the effects of business segment reclassifications in the first quarter of 2015), which is incorporated by reference into this joint proxy statement/prospectus. The selected historical consolidated financial data of Humana for each of the years ended December 31, 2011 and 2010, and as of December 31, 2012, 2011 and 2010, have been derived from Humana’s audited consolidated financial statements for such years, which have not been incorporated by reference into this joint proxy statement/prospectus.

The selected historical consolidated financial data of Humana as of, and for the six months ended, June 30, 2015 and for the six months ended June 30, 2014, are derived from Humana’s unaudited consolidated financial statements and related notes contained in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2015, which is incorporated by reference into this joint proxy statement/prospectus. The selected historical consolidated financial data of Humana as of June 30, 2014 are derived from Humana’s unaudited consolidated financial statements and related notes contained in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2014, which has not been incorporated by reference into this joint proxy statement/prospectus. Humana’s management believes that Humana’s unaudited consolidated financial statements have been prepared on a basis consistent with its audited financial statements and include all normal and recurring adjustments necessary for a fair presentation of the results for each interim period.

You should read the following selected historical consolidated financial data of Humana in conjunction with Humana’s audited consolidated financial statements contained in its Annual Report on Form 10-K for the year ended December 31, 2014, as updated by Humana’s Current Report on Form 8-K filed with the SEC on August 10, 2015 (which, including its related exhibits, updated certain information included in Humana’s Annual Report on Form 10-K for the year ended December 31, 2014, including Items 1, 6, 7 and 8, as it relates to the effects of business segment reclassifications in the first quarter of 2015), and unaudited consolidated financial statements and related notes contained in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2015.

COMPARATIVE HISTORICAL AND UNAUDITED PRO FORMA PER SHARE DATA

The following table sets forth selected historical and unaudited pro forma combined per share information for Aetna and Humana.

Historical Per Share Information of Aetna and Humana. The historical per share information of each of Aetna and Humana below is derived from the audited consolidated financial statements of each of Aetna and Humana as of, and for the year ended, December 31, 2014, and the unaudited consolidated financial statements of each of Aetna and Humana as of, and for the six months ended, June 30, 2015.

Unaudited Pro Forma Combined per Aetna Common Share Data. The unaudited pro forma combined per Aetna common share data set forth below give effect to the mergers under the acquisition method of accounting, as if the mergers had been effective on January 1, 2014, the first day of Aetna’s fiscal year ended December 31, 2014, in the case of net income per share. The unaudited pro forma combined book value per Aetna common share data set forth below give effect to the mergers under the acquisition method of accounting, as if the mergers had been effective June 30, 2015, assuming that each outstanding share of Humana common stock, the Humana RSUs and the Humana PSUs had been converted into Aetna common shares based on the exchange ratio.

The unaudited pro forma combined per Aetna common share data is derived from the audited consolidated financial statements of each of Aetna and Humana as of, and for the year ended, December 31, 2014, and the unaudited condensed consolidated financial statements of each of Aetna and Humana as of, and for the six months ended, June 30, 2015.

The acquisition method of accounting is based on Financial Accounting Standards Board, Accounting Standards Codification (which is referred to in this joint proxy statement/prospectus as ASC) 805, Business Combinations, and uses the fair value concepts defined in ASC 820, Fair Value Measurements and Disclosures, which Aetna has adopted as required. Acquisition accounting requires, among other things, that most assets acquired and liabilities assumed be recognized at their fair values as of the acquisition date. Fair value measurements recorded in acquisition accounting are dependent upon certain valuation studies of Humana’s assets and liabilities and other studies that have yet to commence or progress to a stage where there is sufficient information for a definitive measurement. Accordingly, the pro forma adjustments reflect the assets and liabilities of Humana at their preliminary estimated fair values. Differences between these preliminary estimates and the final values in acquisition accounting will occur, and these differences could have a material impact on the unaudited pro forma combined per share information set forth in the following table.

The unaudited pro forma combined per Aetna common share data does not purport to represent the actual results of operations that Aetna would have achieved had the companies been combined during these periods or to project the future results of operations that Aetna may achieve after completion of the mergers.

Unaudited Pro Forma Combined per Humana Equivalent Share Data. The unaudited pro forma combined per Humana equivalent share data set forth below shows the effect of the mergers from the perspective of an owner of Humana common stock. The information was calculated by multiplying the unaudited pro forma combined per Aetna common share amounts by the exchange ratio.

Generally. You should read the below information in conjunction with the selected historical consolidated financial data included elsewhere in this joint proxy statement/prospectus and the historical consolidated financial statements of Aetna and Humana and related notes that have been filed with the SEC, certain of which are incorporated by reference into this joint proxy statement/prospectus. See “Selected Historical Consolidated Financial Data of Aetna”, “Selected Historical Consolidated Financial Data of Humana” and “Where You Can Find More Information” beginning on pages 38, 40 and 229, respectively, of this joint proxy statement/prospectus. The unaudited pro forma combined per Aetna common share data and the unaudited pro forma combined per Humana equivalent share data is derived from, and should be read in conjunction with, the Aetna

and Humana unaudited pro forma condensed combined financial statements and related notes included in this joint proxy statement/prospectus. See “Aetna and Humana Unaudited Pro Forma Condensed Combined Financial Statements” beginning on page 44 of this joint proxy statement/prospectus.

AETNA AND HUMANA UNAUDITED PRO FORMA CONDENSED

COMBINED FINANCIAL STATEMENTS

The unaudited pro forma condensed combined statements of income for the year ended December 31, 2014, and for the six months ended June 30, 2015, combine the historical consolidated statements of income of Aetna and Humana, giving effect to the mergers as if they had occurred on January 1, 2014, the first day of the fiscal year ended December 31, 2014. The unaudited pro forma condensed combined balance sheet as of June 30, 2015, combines the historical consolidated balance sheets of Aetna and Humana, giving effect to the mergers as if they had occurred on June 30, 2015. The historical consolidated financial information has been adjusted in the unaudited pro forma condensed combined financial statements to give effect to pro forma events that are (i) directly attributable to the mergers, (ii) factually supportable, and (iii) with respect to the statements of income, expected to have a continuing impact on the combined company’s results. The unaudited pro forma condensed combined financial statements should be read in conjunction with the accompanying notes to the unaudited pro forma condensed combined financial statements. In addition, the unaudited pro forma condensed combined financial information was based on, and should be read in conjunction with, the following historical consolidated financial statements and accompanying notes, which are incorporated by reference into this joint proxy statement/prospectus:

The unaudited pro forma condensed combined financial information has been prepared by Aetna using the acquisition method of accounting in accordance with U.S. generally accepted accounting principles, which are referred to in this joint proxy statement/prospectus as GAAP. Aetna has been treated as the acquirer in the mergers for accounting purposes. The acquisition accounting is dependent upon certain valuation and other studies that have yet to commence or progress to a stage where there is sufficient information for a definitive measurement. The mergers have not yet received the necessary approvals from governmental authorities. Under the HSR Act and other relevant laws and regulations, before completion of the mergers, there are significant limitations regarding what Aetna can learn about Humana. The assets and liabilities of Humana have been measured based on various preliminary estimates using assumptions that Aetna believes are reasonable based on information that is currently available to it. Differences between these preliminary estimates and the final acquisition accounting will occur, and those differences could have a material impact on the accompanying unaudited pro forma condensed combined financial statements and the combined company’s future results of operations and financial position. The pro forma adjustments are preliminary and have been made solely for the purpose of providing unaudited pro forma condensed combined financial statements prepared in accordance with the rules and regulations of the SEC.

Aetna intends to commence the necessary valuation and other studies required to complete the acquisition accounting promptly upon completion of the mergers and will finalize the acquisition accounting as soon as practicable within the required measurement period prescribed by ASC 805, but in no event later than one year following completion of the mergers.

The unaudited pro forma condensed combined financial information has been presented for informational purposes only. The unaudited pro forma condensed combined financial information does not purport to represent the actual results of operations that Aetna and Humana would have achieved had the companies been combined during the periods presented in the unaudited pro forma condensed combined financial statements and is not intended to project the future results of operations that the combined company may achieve after the mergers. The unaudited pro forma condensed combined financial information does not reflect any potential divestitures that may occur prior to, or subsequent to, the completion of the mergers or cost savings that may be realized as a result of the mergers and also does not reflect any restructuring or integration-related costs to achieve those potential cost savings. In addition, the unaudited pro forma condensed combined financial information does not reflect any potential debt repayments to reduce Aetna’s debt-to-capital ratio below 40 percent over the 24 months following the completion of the mergers. No material intercompany transactions between Aetna and Humana during the periods presented in the unaudited pro forma condensed combined financial statements have been identified at this time.

Unaudited Pro Forma Condensed Combined Statement of Income For the Year Ended December 31, 2014

See the accompanying notes to the unaudited pro forma condensed combined financial statements, which are an integral part of these statements. The pro forma adjustments shown above are explained in Note 6. Income Statement Pro Forma Adjustments, beginning on page 54 of this joint proxy statement/prospectus.

Unaudited Pro Forma Condensed Combined

Statement of Income

For the Six Months Ended June 30, 2015

See the accompanying notes to the unaudited pro forma condensed combined financial statements, which are an integral part of these statements. The pro forma adjustments shown above are explained in Note 6. Income Statement Pro Forma Adjustments, beginning on page 54 of this joint proxy statement/prospectus.

Unaudited Pro Forma Condensed Combined

Balance Sheet

As of June 30, 2015