Respiratory Syncytial Virus (RSV) remains the leading cause of hospitalization in U.S. infants, placing significant strain on healthcare systems and causing considerable distress for families. The recent U.S. Food and Drug Administration (FDA) approval of Enflonsia, a novel monoclonal antibody developed by Merck, marks a pivotal advancement in the prophylactic management of RSV in this vulnerable population. This single 105-milligram intramuscular injection is designed to provide comprehensive protection for newborns and infants throughout their crucial first RSV season, typically spanning from fall through early spring.
The efficacy of Enflonsia is underscored by robust clinical trial data. The treatment demonstrated an impressive reduction of over 84% in RSV-related hospitalizations in infants compared to placebo. Furthermore, it significantly curtailed the incidence of serious lower respiratory tract infections, such as pneumonia, by more than 60%. These results are a testament to Enflonsia’s potential to substantially mitigate the burden of RSV-associated morbidity and mortality in infants.
As Dr. Dean Li, president of Merck Research Laboratories, affirmed in a statement, “Enflonsia provides an important new preventive option to help protect healthy and at-risk infants born during or entering their first RSV season.” Merck has committed to ensuring the availability of Enflonsia in the U.S. prior to the upcoming RSV season, aiming to alleviate the significant impact of this widespread seasonal infection on both families and healthcare infrastructure.
Adverse events observed in the clinical trials were generally mild and transient, primarily consisting of injection site swelling and rash, with a safety profile comparable to that of placebo. Before Enflonsia can be widely administered, it awaits recommendation from the U.S. Centers for Disease Control and Prevention (CDC). While the CDC’s Advisory Committee on Immunization Practices was slated to review the shot this month, this process has experienced an unforeseen delay following recent changes in the panel’s composition by the U.S. Health and Human Services Secretary. Despite this delay, the medical community eagerly anticipates the full integration of Enflonsia into RSV prevention strategies.
RSV’s ubiquitous nature means most children contract the virus by age two. While older children and adults typically experience mild, cold-like symptoms, infants are particularly susceptible to severe manifestations due to their diminutive airways, which can become severely inflamed, impeding respiration. This can progress to bronchiolitis or pneumonia, and in tragic cases, even lead to infant mortality. Annually, 2% to 3% of U.S. infants under six months of age require hospitalization due to RSV, contributing to an estimated 58,000 to 80,000 hospitalizations in children under five years old each year.
The lack of specific antiviral treatments for RSV necessitates supportive care, often involving oxygen supplementation, as noted by Dr. Amy Edwards, a pediatric infectious disease expert at UH Rainbow Babies and Children’s Hospital in Cleveland. The advent of prophylactic agents like Enflonsia offers a much-needed paradigm shift from reactive treatment to proactive prevention.
Enflonsia: The Impact of RSV on Black Infants
It is crucial for healthcare professionals to recognize and address the disproportionate impact of RSV on Black infants. Data consistently show that Black infants experience higher rates of RSV-related hospitalizations and more severe outcomes compared to their White counterparts.
Enflonsia joins a growing arsenal of RSV preventive measures, including Beyfortus (nirsevimab), another monoclonal antibody approved in 2023, and maternal RSV vaccination during pregnancy. While Beyfortus experienced supply shortages during the last RSV season, the availability of multiple options, including Enflonsia, is vital for ensuring consistent access to protection. Early evidence, such as a CDC study from May 2024, suggests that the combined impact of these new tools is already yielding positive results, with lower RSV hospitalization rates in infants under seven months during the 2024–2025 season compared to previous years.
Dr. Edwards’ optimistic outlook — that widespread uptake of these preventive strategies could “empty us out” of the usual RSV season surge — underscores the transformative potential of Enflonsia and its counterparts in safeguarding infant health and promoting health equity for all.