As Enflonsia nears the first anniversary of its FDA nod, Merck is strengthening its argument for expanding the drug’s label in a move that could allow the preventive antibody to more directly compete with Sanofi and AstraZeneca’s infant RSV protection, Beyfortus.
Updated findings from the late-stage Smart study, which supported Enflonsia’s original approval based on interim data released last June, indicate that the antibody was well tolerated in children under age 2 who remained vulnerable to severe RSV entering their second RSV season and received the therapy at the outset of that period, Merck said in a February 19 announcement.
Merck noted that the safety profile observed in these children aligned closely with results previously reported in infants who were treated during their first RSV season.
In addition, treated participants achieved monoclonal antibody blood levels comparable to those seen in healthy infants enrolled in a separate phase 2b/3 trial known as Clever. According to the company, the Smart data supports extending efficiency expectations to the higher-risk population evaluated in the study.
The Smart trial focused on infants and young children considered at elevated risk for severe RSV because of factors such as premature birth, chronic lung disease related to prematurity or significant congenital heart disease, tracking outcomes across two RSV seasons.
Merck plans to submit the findings, which were presented this week at a Respiratory Syncytial Virus Foundation meeting in Rome, to the FDA and other regulatory agencies as it seeks approval to broaden Enflonsia’s indication to cover high-risk children during their second RSV season.
Currently, Enflonsia is approved only for use in newborns and infants entering their first RSV season. In contrast, Beyfortus (marketed by AstraZeneca and Sanofi) has secured FDA authorization not only for infants in their initial RSV season but also for children up to 2 years old who remain at high risk for severe disease during a second season.
Merck received U.S. approval for Enflonsia last summer, positioning the product for rollout during the ongoing RSV season, which generally spans November through April. When the company released its full-year 2025 financial results earlier this month, it did not separately disclose revenue figures for Enflonsia.
The New Jersey-based pharmaceutical company is likely aiming to replicate the commercial momentum Beyfortus has achieved since launch. Following its first FDA clearance in July 2023, Beyfortus has delivered strong growth for Sanofi and AstraZeneca, with sales rising nearly 10% last year to reach approximately $2.1 billion in 2025.
Even so, potential challenges could be emerging for infant RSV preventives such as Enflonsia and Beyfortus, particularly in the U.S. market.
In December, the FDA informed Merck, AstraZeneca and Sanofi that it had initiated a safety review of their RSV products. At the time, a Merck spokesperson said the company supported the agency’s inquiry and emphasized that the standards for demonstrating the safety of vaccines and preventive monoclonal antibodies such as Enflonsia are extremely rigorous, as they rightly should be.
Merck Eyes Expanded FDA Approval for Englonsia
Merck is moving forward with regulatory efforts to secure broader approval for Englonsia, aiming to extend its protection beyond the first RSV season. The decision marks a significant step for Merck as it continues to strengthen its respiratory portfolio and address unmet needs in infant health.
Respiratory syncytial virus (RSV) remains a leading cause of hospitalization among infants worldwide. While current preventive strategies primarily focus on protection during the first RSV season, Merck believes extended coverage could offer enhanced safety for high-risk populations entering subsequent seasons.
Merck’s Strategy for Extended Protection
According to company statements, Merck has submitted additional clinical data to the U.S. Food and Drug Administration supporting the safety and durability of Englonsia. If approved, Merck would be able to market Englonsia for broader seasonal coverage, potentially redefining RSV prevention standards.
The move positions Merck in a competitive RSV market, where long-acting antibodies and vaccines are reshaping pediatric infectious disease management. By expanding Englonsia’s indication, Merck aims to improve health outcomes and reduce RSV-related hospital admissions.
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
- Editorial Team
