FDA Accepts sBLA for ENFLONSIA, a Minor Growth Pillar Amid Keytruda Transition

What happened

Merck announced FDA acceptance of a supplemental Biologics License Application for ENFLONSIA (clesrovimab) to expand its RSV prevention indication to children under two at increased risk for their second season. While this adds an incremental growth vector, the deep value master report underscores that Keytruda remains 49% of sales and faces a 2028-2029 biosimilar and IRA cliff, dominating the investment case. The report rates MRK a WAIT given a ~$2.5 billion FY2026 headwind from generics and price setting and an unresolved Gardasil shipment pause in China. ENFLONSIA’s pediatric RSV opportunity is a niche relative to the oncology franchise, so the news does not alter the near-term earnings bridge or the pressing need for pipeline delivery before 2028. Investors should monitor the April 28, 2026 Keytruda PDUFA date and the expected Q2 close of the Terns acquisition as more consequential catalysts.

Implication

ENFLONSIA expands Merck's RSV franchise modestly, but the investment thesis remains anchored on whether the company can offset a ~$2.5 billion headwind in 2026 and prove that non-Keytruda pillars can scale before the 2028-2029 exclusivity losses. Without a resolution on Gardasil China resumption or a beat on the erosion bridge, this regulatory milestone is unlikely to shift the stock's risk/reward meaningfully.

Thesis delta

No change to the overall thesis. ENFLONSIA’s sBLA acceptance is a positive pipeline event, but it is far too small to offset the dominance of Keytruda’s impending patent cliff or the ongoing Gardasil China uncertainty that define the investment case. The WAIT rating remains appropriate.