ACHN 2018 Proxy Statement

If you vote your shares over the Internet, only your latest Internet vote will be counted at the Annual Meeting. Attendance at the Annual Meeting does not in itself constitute the revocation of a proxy. Stockholders who have instructed their broker to vote their shares of common stock must follow their broker’s directions in order to change those instructions. You may also attend the Annual Meeting in person instead of submitting a proxy; however, please see the instructions below under “Voting of Shares Held in Street Name” if you wish to vote such shares in person at the Annual Meeting. How Your Proxy Will Be Voted In the absence of specific instructions to the contrary, shares represented by properly executed proxies received by Achillion, including unmarked proxies, will be voted to approve Proposals 1 through 4. In addition, if any other matters properly come before the Annual Meeting, it is the intention of the persons named in the enclosed proxy card to vote the shares they represent as directed by the Board of Directors. We have not received notice of any other matters that may properly be presented at the Annual Meeting. Quorum The presence, in person or by proxy, of the holders of a majority of the outstanding shares of Achillion’s common stock as of the Record Date is necessary to constitute a quorum at the Annual Meeting. As there were 138,339,738 shares eligible to vote on the Record Date, we will need more than 69,169,869 shares present in person or by proxy at the Annual Meeting for a quorum to exist. Voting Tabulation For purposes of determining whether a quorum exists, (i) shares held by stockholders entitled to vote who are present at the Annual Meeting in person or by proxy, (ii) abstentions and (iii) broker non-votes are counted as present or represented at the meeting. An automated system administered by our transfer agent tabulates the votes. Voting Instructions The following section summarizes important information on how to vote your shares of common stock. Voting by Proxy Voting of Shares Registered in Your Name. If you are a record holder, meaning your shares are registered in your name, you may vote over the Internet, by mail or in person at the Annual Meeting pursuant to the following instructions: Over the Internet: Go to the website of our tabulator, Computershare Investor Services, at www.investorvote.com/achn. Use the vote control number printed on your enclosed proxy card to access your account and vote your shares. You must specify how you want your shares voted or your Internet vote cannot be completed, and you will receive an error message. Your shares will be voted according to your instructions. You must submit your Internet proxy before 11:59 p.m. Eastern Time on May 30, 2018 for your proxy to be valid and your vote to count. By Mail: Complete and sign your enclosed proxy card and mail it in the enclosed postage prepaid envelope to Computershare Investor Services. Your shares will be voted according to your instructions. If you do not specify how you want your shares voted, they will be voted as recommended by our Board of Directors. Computershare must receive your proxy card no later than May 30, 2018 for your proxy to be valid and your vote to count. 3