Merck catches a long-acting flu drug for $9.2B to replace vaccines

Merck & Co. (MSD) is acquiring Cidara Therapeutics for $9.2 billion to gain CD388, a potential game-changing flu drug designed to provide season-long protection against influenza.

Why it matters: This deal positions Merck to fill future revenue gaps as Keytruda’s patent expires, while potentially reshaping flu prevention with a more effective and convenient alternative to current vaccines.

Backstory: CD388 is a long-acting antiviral combining a small molecule and protein fragment. In mid-stage trials, it prevented flu symptoms in about 75% of patients over six months. The drug is now in late-stage testing and has received a fast-track FDA designation.

Big picture: Flu vaccines are losing ground amid declining uptake and inconsistent effectiveness, and CD388 covers both influenza A and B strains. A proven, long-lasting antiviral could disrupt a $3.8B market and offer a lifeline to high-risk individuals and vaccine-hesitant populations.

Zoom in: October 2025 marked a high point for biotech acquisitions and signals broader investor interest in innovative treatments. Merck is an enthusiastic participant given that this is its second multibillion-dollar biotech deal this year, following its $10B Verona Pharma buyout. Merck is paying a 109% premium for Cidara shares, and the Financial Times reported that other companies were interested in the acquisition.