Exhibit (a)(1)(C)

Offer to Purchase

All Outstanding Shares of Common Stock

of

ADVERUM BIOTECHNOLOGIES, INC.

at

$3.56 per share, net to the stockholder in cash, without interest and less any applicable tax withholding,

plus one non-tradable contingent value right (“CVR”) per share,

which represents the contractual right to receive up to two contingent cash payments of up to an aggregate of $8.91 per CVR, net to the

stockholder in cash, without interest and less any applicable tax withholding, upon the achievement of

certain specified milestones

Pursuant to the Offer to Purchase dated November 7, 2025

by

FLYING TIGERS ACQUISITION CORPORATION

a direct wholly-owned subsidiary

of

ELI LILLY AND COMPANY

THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT ONE MINUTE PAST 11:59 P.M.,

EASTERN TIME, ON DECEMBER 8, 2025, UNLESS THE OFFER IS EXTENDED OR EARLIER

TERMINATED.

November 7, 2025

To Brokers, Dealers, Commercial Banks, Trust Companies and Other Nominees:

We have been engaged by Flying Tigers Acquisition Corporation, a Delaware corporation (“Purchaser”) and a direct wholly-owned subsidiary of Eli Lilly and Company, an Indiana corporation (“Lilly”), to act as information agent (the “Information Agent”) in connection with the Purchaser’s offer to purchase all of the issued and outstanding shares of common stock, par value $0.0001 per share (the “Shares”), of Adverum Biotechnologies, Inc., a Delaware corporation (“Adverum”), in exchange for (i) $3.56 per Share, net to the stockholder in cash, without interest (the “Cash Consideration”) and less any applicable tax withholding, plus (ii) one non-tradable CVR per Share, which represents the contractual right to receive up to two contingent cash payments of up to an aggregate of $8.91 per CVR, net to the stockholder in cash, without interest and less any applicable tax withholding, upon the achievement of certain specified milestones, all in accordance with the terms and subject to the conditions and other provisions of a contingent value rights agreement to be entered into by and among Lilly, Computershare, Inc., a Delaware corporation, and its affiliate, Computershare Trust Company, N.A., a federally chartered trust company (the Cash Consideration plus one CVR, collectively, the “Offer Price”), upon the terms and subject to the conditions set forth in the Offer to Purchase, dated November 7, 2025 (the “Offer to Purchase”), and in the related Letter of Transmittal (which, together with the Offer to Purchase, as each may be amended or supplemented from time to time, collectively constitute the “Offer”). Please furnish copies of the enclosed materials to those of your clients for whom you hold Shares registered in your name or in the name of your nominee. The CVR Agreement contains important terms, conditions and other provisions relating to any potential payment in respect of the CVRs. It is possible that no payment or a payment of less than $8.91 per CVR will become payable in respect of the CVRs. See Section 11—“The Merger Agreement; Other Agreements—CVR Agreement” of the Offer to Purchase.

The Offer and the Merger are not subject to any financing condition. The conditions to the Offer are described in Section 15 of the Offer to Purchase.

For your information and for forwarding to your clients for whom you hold Shares registered in your name or in the name of your nominee, we are enclosing the following documents:

1. The Offer to Purchase;

2. The Letter of Transmittal (including Internal Revenue Service Form W-9) for your use in accepting the Offer and tendering Shares and for the information of your clients;

3. A form of letter which may be sent to your clients for whose accounts you hold Shares registered in your name or in the name of your nominee, with space provided for obtaining such clients’ instructions with regard to the Offer; and

4. Adverum’s Solicitation/Recommendation Statement on Schedule 14D-9.

We urge you to contact your clients as promptly as possible. Please note that the Offer and withdrawal rights will expire at one minute past 11:59 p.m., Eastern Time, on December 8, 2025 (the “Expiration Time”), unless the expiration of the Offer is extended to a subsequent date in accordance with the terms of the Merger Agreement (as defined below), in which case the term “Expiration Time” means such subsequent time on such subsequent date. Purchaser is not providing for guaranteed delivery procedures. Therefore, Adverum stockholders must allow sufficient time for the necessary tender procedures to be completed during normal business hours of The Depositary Trust Company (“DTC”), which end earlier than the Expiration Time. Normal business hours of DTC are between 8:00 a.m. and 5:00 p.m., Eastern Time, Monday through Friday.

The Offer is being made pursuant to an Agreement and Plan of Merger, dated October 24, 2025 (as it may be amended from time to time, the “Merger Agreement”), by and among Adverum, Lilly and Purchaser, pursuant to which, after consummation of the Offer and subject to the satisfaction or waiver of certain conditions, Purchaser will merge with and into Adverum pursuant to Section 251(h) of the General Corporation Law of the State of Delaware (the “DGCL”), upon the terms and subject to the conditions set forth in the Merger Agreement, with Adverum continuing as the surviving corporation and becoming a direct wholly-owned subsidiary of Lilly (the “Merger”). At the effective time of the Merger (the “Effective Time”), each Share issued and outstanding immediately prior to the Effective Time (other than (i) Shares held in the treasury of Adverum or owned by Adverum, (ii) Shares owned by Lilly, Purchaser or any direct or indirect wholly-owned subsidiary of Lilly or Purchaser or (iii) Shares that are held by stockholders who are entitled to demand and properly exercised and perfected their respective demands for appraisal for such Shares in accordance with Section 262 of the DGCL) will be converted into the right to receive the Offer Price, without interest, from Purchaser, less any applicable tax withholding.

The Board of Directors of Adverum unanimously (i) determined that the Merger Agreement and the transactions contemplated thereby (the “Transactions”) are advisable, fair to, and in the best interests of, Adverum and the holders of the Shares, (ii) duly authorized and approved the execution and delivery of the Merger Agreement, the performance by Adverum of its covenants and other obligations thereunder, and the consummation of the Transactions upon the terms and subject to the conditions set forth therein, (iii) resolved that the Merger Agreement and the Transactions will be governed by and effected under Section 251(h) and other relevant provisions of the DGCL and (iv) resolved to recommend that Adverum stockholders accept the Offer and tender their Shares pursuant to the Offer.

For Shares to be validly tendered to the Purchaser pursuant to the Offer, Computershare Trust Company, N.A., the depositary and paying agent for the Offer (in such capacity, the “Depositary”), must be in timely receipt of (i) to the extent the Shares are not already held with the depositary, the certificates evidencing such Shares (the “Share Certificates”) or confirmation of a book-entry transfer of such Shares into the Depositary’s account at DTC pursuant to the procedures set forth in Section 3 of the Offer to Purchase, (ii) the Letter of Transmittal, properly completed and duly executed, with any required signature guarantees (or, in the case of a book-entry

transfer or a tender through DTC’s Automated Tender Offer Program, an Agent’s Message (as defined in the Offer to Purchase) in lieu of the Letter of Transmittal) and (iii) any other documents required by the Letter of Transmittal or the Depositary, in each case prior to the Expiration Time. Accordingly, tendering stockholders may be paid at different times depending upon when the Share Certificates and Letter of Transmittal, or Book-Entry Confirmations and Agent’s Message, in each case, with respect to Shares that are actually received by the Depositary.

Neither Lilly nor Purchaser will pay any fees or commissions to any broker, dealer, commercial bank, trust company or to any other person (other than to the Depositary and the Information Agent as described in the Offer to Purchase) in connection with the solicitation of tenders of Shares pursuant to the Offer. Brokers, dealers, commercial banks and trust companies will, upon request, be reimbursed by Purchaser for customary mailing and handling expenses incurred by them in forwarding offering materials to their customers. Tendering stockholders who are holders of record of their Shares and who tender directly to the Depositary will not be obligated to pay brokerage fees or commissions or, except as otherwise provided in Section 6 of the Letter of Transmittal, stock transfer taxes with respect to the purchase of Shares by Purchaser pursuant to the Offer.

Questions or requests for assistance may be directed to the Information Agent at the address and telephone numbers listed below. Additional copies of the Offer to Purchase, the related Letter of Transmittal and other materials related to the Offer may be obtained at no cost to stockholders from the Information Agent. Additionally, copies of the Offer to Purchase, the related Letter of Transmittal and any other materials related to the Offer are available free of charge at www.sec.gov.

Very truly yours,

GEORGESON LLC

Nothing contained herein or in the enclosed documents shall render you, the agent of Lilly, Purchaser, the Information Agent, the Depositary or any affiliate of any of them or authorize you or any other person to use any document or make any statement on behalf of any of them in connection with the Offer other than the enclosed documents and the statements contained therein.

The Information Agent for the Offer is:

Georgeson LLC

51 West 52nd Street, 6th Floor

New York, NY 10019

Shareholders, Banks and Brokers

Toll Free: (888) 446-9207

Outside the U.S. and Canada: (862) 243-7027

Email: Adverumoffer@georgeson.com