In the first respiratory syncytial virus (RSV) season with widespread availability of two preventive options against the virus, RSV-related hospitalizations of young infants declined substantially, and both products showed real-world effectiveness similar to what was seen in clinical trials, according to two studies presented at the Infectious Disease Week (IDWeek) 2025 Annual Meeting in Atlanta.

No prophylactic options existed to prevent RSV in the pediatric general population until a maternal RSV vaccine and the monoclonal antibody nirsevimab were approved in 2023. In clinical trials, the maternal vaccine reduced the risk for RSV-related hospitalization by 68% within the first 3 months after birth and by 57% within the first 6 months. Nirsevimab was 83.2% effective against infants’ RSV-related hospitalizations in its pivotal clinical trial.

During the first subsequent RSV season in 2023-2024, there was limited product availability and low uptake as people learned about the options. The 2024-2025 RSV season was therefore the first full RSV season with widely available preventive options, making it now possible to assess their impact on preventing severe disease and hospitalizations in children.

Paul Spearman, MD, director of infectious disease at Cincinnati Children’s Hospital in Cincinnati, was not involved with either study but said the data presented sent a clear message.

“The biggest takeaway is that both the maternal vaccination and the monoclonal antibody administration have real-world effectiveness in preventing RSV hospitalizations and RSV disease [that’s] medically attended,” Spearman told Medscape Medical News. “The initial studies are all done in really well-controlled situations, and now some of these studies are really showing what happens in the real world once you release this product.” 

Spearman said it’s important not only for pediatricians but also for ob/gyns, family doctors, and the medical community more broadly to know how effective these products are.

“The impact on healthcare costs can potentially be tremendous because the hospitalizations are down so significantly,” Spearman added.

Jennifer Schuster, MD, professor of medicine at Children’s Mercy, Kansas City, Missouri, presented data comparing RSV-association hospitalization rates in infants less than 6 months old between the period of 2017-2020 and that of the 2024-2025 season.

The data came from the New Vaccine Surveillance Network (NVSN), which prospectively enrolls children younger than 18 years old who have an emergency, outpatient, or inpatient visit for acute respiratory illness at one of the seven included sites in Rochester, Pittsburgh, Cincinnati, Nashville, Kansas City, Houston, and Seattle. The researchers analyzed trends from infants less than 6 months old who had a positive RSV PCR test, including those with other viral coinfections.

Using hospitalizations per 1000 infants, they calculated relative rate reductions between 2017-2020 and 2024-2025, adjusting for enrollment rates, surveillance days per week, test sensitivity, and hospital market share.

The youngest infants had the greatest reduction in RSV-associated hospitalizations. For babies up to 2 months old, the rate fell from 29.2% to 12.7%, a significant 56% relative rate reduction. In babies 3-5 months old, the rate fell from 16.3% to 11.2%, a 31% significant reduction. Rates fell for all infants less than 6 months old at all seven sites, with relative rate reductions ranging from 39% to 77% depending on the location.

“Widespread availability of RSV prevention products has the potential to dramatically reduce the RSV-associated burden of hospitalizations in young infants,” Schuster told attendees, though the differences in impact varied by location.

Natasha B. Halasa, MD, professor of pediatrics at the Vanderbilt University Medical Center in Nashville, Tennessee, and her colleagues also used data from the NVSN and analyzed the effectiveness of the maternal RSV vaccine in infants younger than 6 months old and the effectiveness of nirsevimab in infants younger than 8 months old. They excluded infants who received both, those who received palivizumab or received nirsevimab within the previous week, and those whose mothers received the vaccine less than 2 weeks before delivery.

Using a test-negative case-control study design, they adjusted their analysis for site, age in months, enrollment month, and having at least one high-risk medical condition. They also adjusted the nirsevimab analysis for insurance and prematurity status, and the maternal vaccine analysis for race and ethnicity.

The population analyzed for the maternal vaccine included 139 RSV-positive cases (19%) and 307 RSV-negative controls (35%). Most (61%) of the 446 infants were hospitalized, 30% were seen in the emergency department (ED), and 9% were seen in outpatient settings. The researchers calculated the vaccine to be 64% effective against any medically attended disease and 70% effective against hospitalization, but there were too few infants to assess effectiveness in any subgroups.

The nirsevimab population of 1795 children included 462 cases (14%) and 1259 controls (44%), with 49% hospitalized, 36% seen in the ED, and 15% seen as outpatients. The researchers calculated the monoclonal antibody to be 76.8% effective against any medically attended RSV, 80.6% effective against hospitalization, and 89.6% effective against ICU admission. Nirsevimab was similarly effective against RSV-A (80.2%) as RSV-B (78.8%), but it was more effective in full-term infants (83.3%) than in preterm infants (69.7%). It was also more effective in infants who received it at birth (87.2%) than in those who received it at a later point (76.6%).

“The estimated maternal RSV vaccine effectiveness represents protection in the first few months of life, and additional studies are needed to evaluate vaccine effectiveness by month of life through at least 6 months of age,” Halasa said. Nirsevimab’s effectiveness, meanwhile, was sustained through at least 5 months, about the length of the RSV season in the US.

Schuster had no disclosures, and her study did not note external funding. Halasa reported receiving research support from Merck, and her study did not note external funding. Spearman reported consulting work for Cyanvac.

Tara Haelle is a science/health journalist based in Dallas.