The US FDA has accepted a supplemental Biologics License Application (sBLA) for Merck’s Enflonsia (Merck is known as MSD outside the United States and Canada).
The application is to protect against respiratory syncytial virus (RSV) in children under two years of age.
Enflonsia is the first and only preventive option against RSV LRTD that can be given to infants without requiring weight-based dosing.
The application is to expand its indication to include protecting children under two years of age from the respiratory syncytial virus (RSV) lower respiratory tract disease (LRTD). It is specifically for those under two who are at increased risk for severe RSV disease through their second RSV season.
Even with protection during their first RSV season, certain children – including those born with chronic lung disease, congenital heart disease or early or moderate preterm birth with certain risk conditions – remain vulnerable to severe RSV disease during their second RSV season.
“For some children, the potentially serious impact of RSV does not stop after their first RSV season,” said Dr Macaya Douoguih, vice president, Therapeutic Area Head, Global Clinical Development, Merck Research Laboratories. “The recent FDA and EMA regulatory filings represent important opportunities to help expand protection with Enflonsia for vulnerable children under two years of age during their second RSV season – potentially further reducing the significant burden of RSV on families and healthcare systems.”
Both the US and EU applications are supported by data from the phase 3 SMART trial, which were presented at the 9th conference of the Respiratory Syncytial Virus Foundation in February 2026 and published in the New England Journal of Medicine in September 2025 (interim RSV season 1 results) and JAMA Pediatrics in July 2026 (full results).
The European Medicines Agency (EMA) accepted the application to expand marketing authorisation for Enflonsia in the EU in July 2026 for the same population.
Enflonsia was approved by the US FDA in June 2025 and by the European Commission in April 2026 for the prevention of RSV LRTD in infants born during or entering their first RSV season.
Enflonsia is approved in more than 40 countries worldwide, including the US, Canada, the EU, China and Japan, for use in infants during their first RSV season. Regulatory filings are underway for approval in additional countries globally, as well as for indication expansion where already approved.
The FDA has set a Prescription Drug User Fee Act (PDUFA), or target action, date of 22 March 2027.