Maternal RSV vaccine and nirsevimab greatly reduce hospitalizations in children, new data shows

Maternal RSV vaccine and nirsevimab greatly reduce hospitalizations in children, new data shows

For the first time, U.S. infants saw dramatically fewer RSV hospitalizations due to new maternal vaccines and nirsevimab, highlighting the urgent need for early, widespread prevention each season.

In a recent report published in theWeekly Morbidity and Mortality ReportA group of researchers evaluated changes in respiratory syncytial virus (RSV) prevalence in children under five years of age following widespread availability of maternal vaccination and nirsevimab during the 2024-202 season.

Because this was an ecological analysis, the researchers did not have individual-level data linking receipt of RSV prevention products to hospitalization with hospitalization outcomes. Therefore, causality cannot be definitively established.

background

RSV is the most common cause of hospitalization in the United States (US), with the highest risk in those under two months of age. Each RSV season places a strain on pediatric healthcare systems, particularly during peak winter months. Until recently, preventive options for RSV were limited. In 2023, two new interventions became available: a maternal RSV vaccine administered during late pregnancy and nirsevimab, a long-acting monoclonal antibody for infants. These tools aim to protect infants during their most vulnerable period. However, real-world data on their effectiveness at the population level remains limited. The report emphasizes the need for further research to assess long-term and equitable outcomes and notes several limitations, including the possibility of incomplete adjustments for under-detection or under-monitoring, the non-national representativeness of surveillance areas, and the preliminary nature of the data.

About the study

This analysis used surveillance data from two national systems: the RSV-associated hospitalization surveillance network (RSV-NET) and the new vaccine surveillance network (NVSN). Both systems conducted active population-based surveillance for laboratory-confirmed RSV-associated hospitalizations in children under five years of age. RSV-NET operated in 13 states and covered 161 counties, while NVSN covered seven metropolitan areas.

Hospitalizations in October 2024 to February 2025 were compared to the pooled pre-coronavirus disease 2019 (Covid-19) (2018-2020). Children were divided into three age groups: 0-7 months (eligible for maternal vaccination or nirsevimab), 8-19 months (some eligible for nirsevimab based on risks), and 20-59 months (ineligible). RSV confirmation was based on reverse transcription polymerase chain reaction or rapid antigen detection within 14 days of admission. Rates were adjusted for factors such as undertesting, test sensitivity, hospital market share and enrollment gaps. Sensitivity analyzes were conducted, including one excluding Houston, Texas, where early viral circulation preceded widespread product use. Statistical analyzes included rate ratios, z-tests, and bootstrap-derived confidence intervals. This activity was reviewed by the Centers for Disease Control and Prevention and classified as public health surveillance without requiring institutional review board approval.

Study results

A total of 18,389 RSV-associated hospitalizations were recorded: 11,681 in 2018-2020 and 6,708 in 2024-2025. Median patient age increased in both networks, indicating that younger infants were better protected in the later period.

Among infants aged 0 to 7 months, the primary target group for prevention, the 2024-2025 season saw significantly lower hospitalizations. In RSV-NET the rate fell from 15.0 to 8.5 per 1,000 children, a reduction of 43%. In NVSN, the rate fell from 14.8 to 10.7, reflecting a reduction of 28%. The most significant improvement was observed in infants aged 0 to 2 months with a reduction of 52% for RSV-NET and 45% for NVSN. When Houston was excluded from the NVSN analysis, the decline in infants aged 0 to 2 months reached 71%.

Children aged 8-19 months and 20-59 months who were generally ineligible or partially eligible for RSV prevention had higher hospitalization rates in 2024-2025 compared to pre-pandemic seasons. For example, RSV-NET showed a 33% increase at 8-19 months and a 64% increase at 20-59 months. These results suggest that the 2024-2025 RSV season was more severe overall and highlighted that the reductions in younger infants were likely due to the prevention products, not viral spread or changes in care trials.

The trends were consistent for both weekly (RSV-NET), monthly (RSV-NET) and monthly (NVSN) time frames, particularly in the peak months of December to February. This timing is consistent with increased uptake of maternal RSV vaccination and nirsevimab. By February 2025, an estimated 66% of infants aged 0 to 7 months have been protected, up from 30% in October.

The results support the effectiveness of both prevention methods in reducing severe disease during peak transmission. In areas where products were used before the virus surge, such as those excluded from the Houston exemption, the benefits appeared to be even greater. These trends reflect the results of the European studies and further validate their global relevance.

The authors' caution that reductions in hospitalizations may be observed may be underestimated given the increased severity of the 2024-2025 season for older, unprotected children and recommend that population-level impacts can be maximized through early, widespread use of RSV prevention products prior to RSV virus transmission.

Respiratory syncytial virus-associated hospitalization rates* in children aged <5 years by age group and week of surveillance - Syncytial virus-associated hospitalization surveillance network, USA, October to April 2018-20 and October - 2024-25

Conclusions

In summary, the first US RSV season with widespread access to maternal vaccination and nirsevimab resulted in significantly fewer hospitalizations in infants aged 0 to 7 months. These declines were most notable during the peak winter months and among the youngest infants, who are at highest risk of complications. Higher hospitalization rates in older, unprotected age groups are likely to increase reductions in preventive products. These results highlight the urgency of early and widespread use of RSV vaccines and maternal nirsevimab. Health systems must prioritize timely delivery of these interventions to protect infants in future RSV seasons and reduce the overall burden of disease.

Because vaccination or prophylactic status was not available at the individual level, the report highlights the need for further surveillance and research to determine the long-term and equitable effectiveness of these prevention strategies.

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