The U.S. Food and Drug Administration has granted approval to Merck’s new vaccine designed to protect infants from respiratory syncytial virus (RSV) during their first season of exposure. This development introduces a new competitor to the existing treatment offered by Sanofi and AstraZeneca.
Merck’s vaccine, known as Enflonsia, is set to be distributed ahead of the typical RSV season, which usually runs from autumn through to spring. The company has announced that orders will begin in July, with shipments planned to ensure the vaccine is available before the virus spreads widely. RSV is a leading cause of hospital admissions among newborns and is responsible for numerous deaths each year, particularly affecting infants and older adults.
This approval provides healthcare professionals with an additional option to combat RSV, which places a heavy burden on families and healthcare systems.
“We are committed to ensuring availability of [Enflonsia] in the U.S. before the start of the upcoming RSV season to help reduce the significant burden of this widespread seasonal infection on families and health care systems,” Dr. Dean Li, president of Merck Research Laboratories, stated.
Enflonsia will compete directly with Beyfortus, a monoclonal antibody treatment from Sanofi and AstraZeneca that faced nationwide shortages during the 2023 RSV season due to unprecedented demand. Both vaccines deliver antibodies directly into the bloodstream to provide immediate protection, but they target different parts of the virus, making direct comparisons challenging.
A key advantage of Merck’s vaccine is its dosing flexibility, as it can be administered to infants regardless of their weight, potentially simplifying the vaccination process. Beyfortus, in contrast, requires dosage adjustments based on the infant’s body weight, which may complicate administration.
In response to previous supply issues, Sanofi has announced plans to increase production and begin earlier shipments of Beyfortus in the third quarter of this year. Last year, Beyfortus generated sales of $1.8 billion.
Although RSV vaccines from companies such as Pfizer, GSK, and Moderna are available in the United States, these are intended only for adults or pregnant women. Recently, the FDA paused clinical trials of RSV vaccines in young children while safety concerns are being evaluated.
All companies involved are awaiting recommendations from an upcoming advisory committee meeting organised by the Centers for Disease Control and Prevention, scheduled from June 25-27. This panel will provide guidance on RSV immunisation and other vaccines.
Clinical trials of Enflonsia have shown strong results. In mid- to late-stage studies, the vaccine reduced RSV-related hospitalisations by more than 84% and hospital admissions due to lower respiratory tract infections by 90% in infants up to five months old, compared with placebo. It also decreased medically attended lower respiratory infections by over 60% in the same age group.
RSV remains a major cause of lower respiratory infections such as pneumonia, especially among infants and the elderly, making effective preventative measures a public health priority.