Timing of RSV Immunization Matters for Infant Protection suggests study

The seasonal timing of when infants receive the new respiratory syncytial virus (RSV) immunization is crucial for ensuring its effectiveness, according to Yale research published in JAMA Network Open.

Nirsevimab is a seasonal immunization that targets RSV in infants. As a monoclonal antibody-a protein that can bind to a specific target-nirsevimab binds to a particular area of the virus and blocks RSV from entering human cells.

In a new study testing the effectiveness of the monoclonal antibody in a real-world setting, researchers at Yale School of Medicine (YSM) found that immunization was up to 85% effective at preventing serious illness in children under one year of age. However, timing its administration to coincide with seasonal outbreaks is important because the monoclonal antibody’s effectiveness waned three months after receiving the shot.

Children with severe RSV “are essentially drowning in their own mucus,” says Carlos Oliveira, MD, PhD, assistant professor of pediatrics (infectious diseases) at YSM and senior author of the study. So having immunizations that can prevent this and keep vulnerable infants from ending up in the hospital “is huge,” he says.

A dangerous disease

RSV is incredibly common. Almost every child will contract the virus before the age of two. Some people who get RSV only develop a runny nose and fever. But for infants and elderly patients, the virus can be deadly.

“Any parents of a baby with severe RSV will see how their child is constantly struggling to breathe,” says Oliveira, who is also an assistant professor of biostatistics at Yale School of Public Health. Excessive mucus produced to fight off the virus can clog the small airways of children under the age of six months. Globally, around 1.4 million children under the age of six months end up hospitalized due to RSV every year, mostly during the fall and winter. Of those, approximately 27,000 will die.

There is no targeted treatment for RSV. But in July 2023, the U.S. Food and Drug Administration (FDA) licensed nirsevimab as the first single-dose, long-acting monoclonal antibody for the prevention of RSV in infants. Previously, palivizumab was the only preventative option and required monthly injections to sustain its protective effect.

Clinical trials found nirsevimab to be safe and effective for infants. This study is an important continuation of that research. Vaccines and medications are still studied after FDA approval to track their effectiveness in the general population-outside of the clinical trial setting-and further inform optimal use. These types of studies are considered phase IV trials.

RSV immunization prevents severe illness

Oliveira and his colleagues collected data on approximately 3,000 infants who received care in the Yale New Haven Health System for respiratory illnesses between October 2023 and May 2024. This was the first RSV season in which nirsevimab was available, and only 10% of infants in the study had received the immunization before seeking medical help.

Of the 3,000 infants, the researchers tracked who had tested positive for RSV, who had been immunized, and how they fared across the study period. About 680 infants tested positive for RSV, and 166 infants-97% of whom had not been vaccinated with nirsevimab-ended up hospitalized.

The data showed that the monoclonal antibody didn’t provide total protection against RSV; nirsevimab was around 68% effective at preventing RSV infection in infants. However, the researchers found that immunization was 85% effective at preventing infants from being hospitalized.

So, while nirsevimab didn’t prevent all RSV infections, “the key takeaway is that the monoclonal antibody was very good at preventing severe disease,” says Oliveira. Immunization was the difference between an infant with a runny nose and one that ended up in the ICU.

Oliveira and his colleagues’ research also shows that nirsevimab becomes less effective after about 14 weeks. The U.S. Centers for Disease Control and Prevention recommends that infants who are eight months old or younger receive the shot at the very start of the RSV season, which runs between October and March.

Information from Oliveira’s study will help policymakers decide when to roll out nirsevimab during the RSV season.

Reference:

Xu H, Aparicio C, Wats A, et al. Estimated Effectiveness of Nirsevimab Against Respiratory Syncytial Virus. JAMA Netw Open. 2025;8(3):e250380. doi:10.1001/jamanetworkopen.2025.0380