Merck bets on a novel long-acting antiviral approach to expand influenza prevention options
Merck deepens respiratory portfolio with acquisition of Cidara and first-in-class long-acting flu Prophylaxis, CD388
Merck & Co have recently announced they will be acquiring Cidara Therapetuics securing full rights to its lead long-acting influenza prophylactic, CD388. CD388 is a strain-agnostic antiviral developed using Cidara’s propriety Cloudbreak drug-Fc conjugate platform, coupling the antiviral zanamivir with a human antibody fragment. The drug is currently in being assessed in individuals at high risk of flu in the Phase 3 ANCHOR trial and was previously granted breakthrough therapy designation by the FDA. Merck’s CEO Robert M Davis highlighted that CD388 has the potential be first-in-class for long-acting antiviral influenza prophylaxis with Jeff Stein, CEO of Cidara adding that CD388 has the potential “to provide an additional option to vaccines and antivirals to help address unmet needs in influenza prevention”. The transaction is expected to close in the first quarter of 2026.
Long-acting Prophylaxis emerges as a potential shift from traditional flu vaccination
This deal appears to be a strategic push by Merck to diversify and strengthen its infectious disease portfolio with CD388 potentially representing a shift in the flu prevention paradigm. Traditional flu strain matched vaccines remain standard but CD388s “once per season” antiviral prophylaxis may offer an alternative or adjunctive offering expanding prevention options beyond vaccines and standard antivirals. The value proposition is potentially strong for high-risk populations such as older adults and the immunocompromised where vaccine manufacturers may face increased pressure to demonstrate improved efficacy and longer duration. More broadly, seasonal mAb prophylaxis is gaining traction in select populations, as seen with AstraZeneca and Sanofi’s Beyfortus and Merck’s own Enflonisa programme, with mAb approaches typically associated with lower reactogenicity than vaccines, an attribute that may be particularly attractive in frail or vaccine-hesitant patients.
Strain-agnostic mechanism and broad eligibility strengthen CD388’s commercial potential
Further, CD388’s strain agnostic approach may overcome the issue of strain matching faced by current flu vaccines, though mRNA vaccines (such as those being developed by Pfizer and Moderna) faster development time has offered a potential improvement in this area. This, coupled with the immune response independent approach could allow for premium pricing and broad eligibility, with Merck noting it expects 110 million individuals in the US alone to be eligible and a market opportunity of $5 billion.
Approval horizon creates window for incumbents to reinforce competitive positioning
With the Phase 3 Anchor trials PCD in Jan 2027, CD388 is not expected to enter the market until 2028 at the earliest and must confirm its positive Phase 2b results in its Phase 3 trial as well as clear regulatory and reimbursement hurdles. This will allow for current key players in the flu prophylaxis such CSL Seqirus, Sanofi and GSK to shore up their strategies, with their own pipeline assets or focus on strengthening positioning by highlighting proven real-world effectiveness, safety, and established public-health impact. Sanofi’s recent expansion of its licensing agreement with Novavax will allow it to use the Matrix-M adjuvant within its influenza vaccine portfolio, creating an opportunity to explore next-generation enhancements to Fluzone High-Dose (HD), the long-established standard of care for older-adult flu prophylaxis. Combining Fluzone HD’s proven immunogenicity with Matrix-M’s capacity to further boost breadth and durability of protection could help Sanofi reinforce differentiation in a key population.
Authors: Yushi Redhead, PhD, Consultant and Eu Shen Seow, PhD, Senior Business Analyst
