Merck Bets on Flu Antiviral with $9.2B Acquisition of Cidara Therapeutics

Merck & Co. has announced that it will acquire Cidara Therapeutics in a deal valued at approximately $9.2 billion. Through the deal, Merck will receive Cidara’s lead Phase III influenza antiviral candidate.

The deal comes in at $221.50 per share, representing a premium of roughly 108% to 110% versus Cidara’s pre-announcement share price.

The acquisition is expected to close in Q1 2026.

Merck is making the move to strengthen its portfolio of respiratory and antiviral offerings.

Cidara’s lead asset, CD388, is a long-acting antiviral designed to provide season-long protection against influenza A and B, including for populations that do not respond well to vaccines (i.e., older adults, immunocompromised individuals).

It belongs to the drug-Fc conjugate class (or “DFC” class) and contains multiple copies of a potent small-molecule neuraminidase inhibitor conjugated to a customized human antibody Fc fragment.

J&J’s pharmaceutical division, Janssen, had initially licensed the global rights to CD388 from Cidara in April 2021. However, in mid-2023, J&J dropped CD388, announcing that it was discontinuing the internal development of most of its infectious disease pipeline to focus on other core therapeutic areas.

Following this, Cidara regained the full global rights to the CD388 program in April 2024 for an upfront payment of $85 million. Cidara then secured significant funding and advanced the drug through successful Phase IIb trials.

Since then, the company has advanced CD388 into the Phase III Anchor trial, backed by a potential $339 million funding agreement with HHS Biomedical Advanced Research and Development Authority (BARDA).

According to Cidara, the deal includes $58 million in guaranteed funding over two years, designated to establish US-based manufacturing for CD388 and to support Cidara’s initial commercial supply chain.

CD388 received Breakthrough Therapy designation from the FDA last month.

“We continue to execute our science-led business development strategy, augmenting our pipeline with CD388, a potentially first-in-class, long-acting antiviral designed to prevent influenza in individuals at higher risk of complications,” said Robert M. Davis, chairman and chief executive officer, Merck.

“We intend to build on the Cidara team’s remarkable progress and are confident that CD388 has the potential to be another important driver of growth through the next decade, creating real value for shareholders.”

The flu prevention market is large and remains an area of unmet need, particularly in “vaccine-cold” years or for patients who cannot mount immune responses.

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CD388’s early data are compelling: in a Phase IIb trial (the NAVIGATE study), at its highest 450-mg dose, CD388 reportedly showed ~76% reduction in symptomatic influenza over 24 weeks versus placebo in healthy adults. Lower doses of 300 mg and 150 mg yielded 61% and 58% protection, respectively.

A successful Phase III readout and label could position CD388 as a “single-dose, season-long” flu prophylactic, potentially transforming the standard of care for at-risk populations.

“This milestone represents a transformational moment for Cidara and for our mission to redefine influenza prevention,” said Jeffrey Stein, PhD, president and chief executive officer of Cidara.

“Thanks to the extraordinary dedication of our team, the Phase IIb NAVIGATE study delivered compelling results that demonstrate CD388’s potential to provide an additional option to vaccines and antivirals to help address unmet needs in influenza prevention. Merck’s global development, regulatory and commercial capabilities provide the expertise and resources needed to bring this important innovation to those individuals who need it most.”

Merck estimates the potential commercial opportunity for CD388 to exceed $5 billion.

The acquisition aligns with Merck’s broader strategy to offset upcoming patent expirations for key assets, notably its blockbuster immunotherapy Keytruda (pembrolizumab), and to expand into new growth avenues through M&A.