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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

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FORM 6-K

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REPORT OF FOREIGN PRIVATE ISSUER PURSUANT TO RULE 13a-16 OR 15d-16 UNDER THE SECURITIES EXCHANGE ACT OF 1934

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For the month of September, 2026

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Commission file number: 001-37891

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AC IMMUNE SA

(Exact Name of Registrant as Specified in Its Charter)

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EPFL Innovation Park

Building B

1015 Lausanne, Switzerland

(Address of Principal Executive Offices)

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Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F.

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Form 20-F ☒     Form 40-F ☐

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On September 23, 2026, AC Immune SA issued a press release announcing positive safety and immunogenicity results through week-100 in Part 1 of VacSYn, its randomized, double-blind, placebo-controlled Phase 2 trial of ACI-7104 in patients with early-stage Parkinson’s disease (PD). A copy of the press release is attached as Exhibit 99.1 to this Report on Form 6-K.

This Report on Form 6-K (excluding Exhibit 99.1) shall be deemed to be incorporated by reference into the registration statements on Form F-3 (File Nos. 333-227016, 333-249655 and 333-277940) and Form S-8 (File Nos. 333-213865, 333-216539 and 333-233019) of AC Immune SA and to be a part thereof from the date on which this report is filed, to the extent not superseded by documents or reports subsequently filed or furnished.

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EXHIBIT INDEX

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SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

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