Merck's $9.2 Billion Play: Acquiring Cidara to Strengthen Influenza Defense Portfolio

The Big Picture

Merck just made a major bet in the antiviral space, agreeing to acquire Cidara Therapeutics for $221.50 per share, translating to approximately $9.2 billion in total deal value. The driving force? CD388, an experimental antiviral compound designed to prevent influenza in high-risk populations. This move marks a significant strategic pivot for Merck, signaling the pharma giant’s commitment to expanding its respiratory and infectious disease offerings beyond traditional approaches.

Why CD388 Matters: A Different Approach to Flu Prevention

What makes this acquisition particularly interesting is CD388 itself. Unlike conventional vaccines, CD388 operates as a long-acting, strain-independent antiviral agent—meaning it can theoretically protect against multiple influenza virus strains without relying on the immune system’s response. This becomes crucial for immunocompromised patients, elderly individuals, and those with chronic conditions who may not respond well to traditional vaccines.

The drug represents a novel category: a drug-Fc conjugate combining a small molecule neuraminidase inhibitor with a human antibody fragment. In layman’s terms, it’s designed to both directly block the virus and trigger immune clearance simultaneously—a dual-mechanism approach that differentiates it from existing treatments.

Clinical Progress and FDA Momentum

CD388 is currently in Phase 3 development, undergoing evaluation in the ANCHOR study across 150 sites in the U.S. and UK with a target enrollment of 6,000 participants. The drug’s advancement was accelerated by compelling results from the Phase 2b NAVIGATE trial, which demonstrated CD388’s ability to prevent symptomatic laboratory-confirmed influenza in healthy unvaccinated adults aged 18-64.

This clinical success earned CD388 FDA Breakthrough Therapy Designation in October 2025, a fast-track status that signals regulatory confidence and expedites the review process. An interim analysis is planned for Q1 2026, with the full Phase 3 results expected to inform the drug’s path to potential market approval.

The Scale of Opportunity

The transaction valuation reflects how seriously Merck views the influenza prevention market. Globally, approximately 1 billion people contract seasonal flu annually, with 3-5 million experiencing severe complications. The WHO estimates 290,000-650,000 annual deaths worldwide from influenza-related illness, including 6,300-52,000 in the U.S. alone. For vulnerable populations—particularly older adults and immunocompromised individuals—the unmet medical need is substantial.

By adding CD388 to its portfolio, Merck gains access to a potentially first-in-class therapy that could offer season-long protection without vaccine-dependent efficacy concerns. This diversifies Merck’s respiratory pipeline while addressing a genuine gap in antiviral options.

Deal Mechanics and Timeline

Under the agreement, Merck will acquire all outstanding Cidara shares through a subsidiary via a tender offer, with shareholder approval required. The transaction is expected to close in Q1 2026, subject to regulatory approvals including Hart-Scott-Rodino antitrust clearance and customary closing conditions. The deal will be accounted for as an asset acquisition.

Strategic Rationale

This acquisition aligns with what Merck calls its science-led business development strategy—essentially, buying validated scientific innovation rather than building from scratch. For Cidara, the deal provides access to Merck’s global development, regulatory, and commercial infrastructure, dramatically accelerating CD388’s potential path to patients. For Merck, it locks in a novel antiviral asset before competitors can acquire comparable capabilities in this increasingly important therapeutic area.

Robert M. Davis, Merck’s chairman and CEO, framed the move as positioning CD388 as “another important driver of growth through the next decade”—suggesting management’s confidence in the drug’s long-term commercial potential and the durability of the influenza prevention market opportunity.