June 10, 2025 – The FDA has approved a new shot to prevent respiratory syncytial virus (RSV) infection in babies during their first RSV season, which typically lasts from fall through spring.
Clesrovimab, which is sold as Enflonsia, is the first and only RSV protection for babies that uses the same dose for all infants, regardless of their body weight. This makes it a simpler and more precise option that ensures reliable protection throughout the RSV season.
RSV is a common virus that spreads during colder months through coughs, sneezes, direct contact, or contaminated surfaces. It typically causes mild, cold-like symptoms. Infants (especially those under 6 months) and older adults are more likely to have a severe RSV infection. RSV can cause bronchiolitis (inflammation of small airways) and pneumonia, sometimes leading to hospitalization. Because it's the leading cause of hospitalization in infants, vaccines and preventive treatments are the best way to protect them.
Merck, the maker of the new preventive, said the FDA's decision was based on two clinical trials. In the first trial, a single dose of Enflonsia was given to newborns, both preterm and full-term, and reduced RSV severe lung infections by 60% and hospitalizations by 84% over five months, compared to a placebo. The second trial showed Enflonsia was as effective and safe as an older RSV medicine (palivizumab) for babies at higher risk, such as those born very early or with lung or heart issues. The clinical trials also showed that Enflonsia was just as safe on its own or with other routine childhood vaccines.
"Enflonsia combines dosing convenience with strong clinical data showing significant reductions in RSV disease incidence and hospitalizations, making it a promising new intervention to help protect infants from RSV," Octavio Ramilo, MD, chair of the Department of Infectious Diseases at St. Jude Children's Research Hospital and investigator for the clinical trials, said in a statement.
The Enflonsia shot contains ready-made antibodies that help newborns and infants fight off RSV before it can cause serious lung infections. It uses a single, non-weight-based, 105-mg dose that gives fast and lasting protection for about five months. Babies born during RSV season should get the shot at birth, while those born before the season should get it just before it starts. The preventive is given as an injection in the baby’s thigh.
Babies will be monitored for any serious allergic reactions after the injection. The most common side effects include redness or swelling where the shot was given, and mild skin rashes.