Giving the long-acting monoclonal antibody nirsevimab to most infants sharply reduced hospitalizations and clinic visits linked to respiratory syncytial virus (RSV) during the 2023–24 respiratory virus season, according to findings published yesterday in The Lancet Infectious Diseases. The study also suggests that the protection extended into the following season. 

The findings arrive as US regulators, without supporting evidence, deemphasize the importance of childhood vaccinations. Earlier this month, the Centers for Disease Control and Prevention (CDC) reduced the number of diseases it recommends to vaccinate against from 17 to just 11. The vaccines removed from the long-established childhood vaccine schedule, including the RSV vaccine, are now recommended only for “high-risk groups.” 

RSV is the leading cause of hospitalization among infants in the United States. As many as 80,000 children aged 5 years or younger with RSV are admitted to a hospital each year, with even greater risk for children born prematurely. 

86% drop in hospital admissions

Led by researchers at the Instituto de Investigacion Sanitaria de Santiago de Compostela in Spain, the population-based analysis followed 12,492 infants eligible for nirsevimab during the 2023–24 RSV season through the end of the following season. Over 94% of eligible infants received the antibody, allowing researchers to assess its population-level impact.

Compared with hospitalizations in RSV seasons before the introduction of widespread nirsevimab immunization (2016–2023, but excluding 2020–21 and 2021–22 owing to the COVID-19 pandemic), RSV-related lower respiratory tract infection (LRTI) hospitalizations fell nearly 86% in 2023–24 and 55% in 2024–25.

Reductions were also seen in first-time hospital admissions for LRTI, acute bronchitis, and bronchiolitis, as well as in outpatient visits for wheezing, asthma, and bronchitis or bronchiolitis. All-cause hospitalization declined approximately 20% in the first season but didn’t decline significantly in the second season. 

In an accompanying commentary, Corinne Levy, MD, and Robert Cohen, MD, both of Paris-East Creteil University in France, described the rollout of nirsevimab during the 2023–24 season as “a major turning point for infants and for health-care systems as a whole,” noting dramatic reductions in RSV bronchiolitis. They emphasized that protection was conferred across two consecutive RSV seasons, despite shifts in dominant viral subtypes (from RSV-A in the first season to RSV-B in the second).

“We have provided the first prospective, population-based evidence that a single dose of nirsevimab, when deployed universally, confers substantial and sustained impact on severe RSV outcomes beyond the first 6 months after administration and across two consecutive RSV seasons,” write the authors. 

43% drop in emergency department visits

Separate findings published this month in Emerging Infectious Diseases add to a growing body of evidence that universal immunization can substantially reduce severe RSV. 

The study, led by researchers at the University of Milan in Milan, Italy, assessed the outcome of a universal RSV immunization campaign in the Lombardy region during the 2024–25 respiratory virus season. The administration of nirsevimab to 79% of eligible infants was associated with significant reductions in emergency department (ED) visits and hospitalizations among children under 12 months. 

Comparing the 2024–25 respiratory virus season with previous seasons, the researchers estimated a 42.7% reduction in ED visits and a 46.5% reduction in hospitalizations for LRTI in infants. Children aged 1 to 5 years, who were ineligible for nirsevimab, did not experience similar declines. 

The researchers credit widespread immunization rather than a less severe RSV season for this effect. “The observed reduction in RSV-related outcomes in infants was not mirrored in children 1–5 years of age,” they write, “suggesting that the intervention likely prevented disease among infants, not that levels of RSV circulation in the population were reduced.”

Premature babies make up 1 in 5 RSV hospitalizations

Preterm children accounted for one in five hospitalizations among those 2 years or younger who were hospitalized with RSV in the United States from 2016 to 2023, according to new findings published in Pediatrics. 

The study, led by researchers from the CDC, analyzed data from nearly 6,000 children. The data also suggest that preterm children under age 2 years are at greater risk for prolonged hospitalization, intensive care unit admission, and assisted ventilation than full-term children. 

“Collectively, these findings document the disproportionate burden of RSV-associated hospitalizations and related complications in premature children,” write the authors. “Currently recommended RSV prevention products, including maternal RSV vaccine, nirsevimab, and clesrovimab, offer effective options for protecting premature infants.”