Background and Objectives of the Study

Respiratory syncytial virus (RSV) bronchiolitis is one of the leading causes of hospitalization among infants. In France, families can choose one of two available approaches to limit severe cases: maternal vaccination with the RSVpreF vaccine and passive immunization with nirsevimab, with efficacy ranging from 50% to 80%. Some infants may have benefited from both, either intentionally (birth less than 14 days after maternal vaccination) or inadvertently.

This study reports on hospitalizations for lower respiratory tract infections associated with RSV in infants who received dual immunization.

Methodology

This study was conducted using data from the National Health Data System (SNDS). It included infants born in France between September 1 and December 31, 2024, who were exposed to both maternal vaccination and nirsevimab. Maternal vaccination was considered optimal if administered between 24 and 36 weeks of gestation, with a minimum interval of 14 days prior to delivery. The outcome measures included hospitalizations for RSV-related lower respiratory tract infections and deaths. The infants were followed up until February 28, 2025.

Results

During the 2024–2025 season, 3,597 of the 297,295 vaccinated children (1.2%) received two doses of the vaccine. Optimal maternal vaccination was observed in 2,602 infants (72%). The majority of children had received nirsevimab by the end of November, before the mid-December epidemic peak that year.

The overall hospitalization rate for RSV-related infection was 0.86% (0.88% for infants whose mothers had received optimal vaccination and 0.80% for those whose mothers had not received optimal vaccination). Among infants whose mothers had received optimal vaccination, hospitalization for RSV-associated lower respiratory tract infections was observed in 2 high-risk infants (0.75%; 95% CI, 0.00–1.78) and in 21 low-risk infants (0.90%; 95% CI, 0.52–1.28). One death occurred among the study population between September 1, 2024, and February 28, 2025.

In a previous study, we demonstrated greater efficacy of passive immunization compared to maternal vaccination. In the group that received passive immunization, the hospitalization rate for RSV–associated lower respiratory tract infections was 1.0% among full–term infants and 1.1% among preterm infants. These figures are similar to the percentages observed in the present study.