Maternal Vaccination vs Nirsevimab Use and Respiratory Syncytial Virus Infection Among Full-Term Infants

Importance Since 2023, infants can be protected from respiratory syncytial virus (RSV) through a vaccine (RSVpreF) for pregnant persons or a monoclonal antibody (nirsevimab) for infants. Objective To assess the relative product effectiveness (rPE) of maternal RSVpreF vs infant nirsevimab against RSV infection and hospitalization among infants born in the 2023 to 2025 RSV seasons. Design, Setting, and Participants This retrospective cohort study included full-term infants born at Kaiser Permanente Northern California (KPNC), an integrated health care system with approximately 40 000 births each year, from December 3, 2023, to March 30, 2024, or September 29, 2024, to March 15, 2025. Infants protected by RSVpreF were matched to infants protected with nirsevimab with optimal full matching (up to 1:5), adjusting for birth week, sex, and maternal factors. Each group was also compared with infants with no protection, matching unprotected infants to both RSVpreF and nirsevimab infants (up to 1:10). Exposures Maternal RSVpreF at 32 to 36 weeks’ gestation or infant nirsevimab through age 7 days. Main Outcomes and Measures Polymerase chain reaction (PCR)–confirmed RSV and RSV hospitalization (PCR-confirmed RSV 10 days before to 7 days after hospitalization) in infants aged 8 to 180 days or through RSV season end (April 30 in both seasons). rPE was estimated from age 8 to 180 days using restricted mean time lost. Results Of 15 124 matched infants (7729 [51.1%] male) in the primary analysis, 146 (0.97%) had RSV infection (106 of 11 293 [0.94%] in RSVpreF group and 40 of 3831 [1.04%] in nirsevimab group) and 12 (0.08%) were hospitalized (7 [0.06%] in RSVpreF group and 5 [0.13%] in nirsevimab group). The rPE of RSVpreF vs nirsevimab was −5.3% (95% CI, −44.8% to 23.4%) against infection and 61.5% (95% CI, −10.8% to 86.7%) against hospitalization. Compared with no protection, RSVpreF rPE was 69.3% (95% CI, 59.9% to 76.6%) against infection and 86.4% (95% CI, 66.4% to 94.5%) against hospitalization; nirsevimab rPE was 65.2% (95% CI, 51.0%-75.4%) and 73.8% (95% CI, 33.4%-89.7%), respectively. Conclusion In this cohort study of infants born in the 2023 to 2024 and 2024 to 2025 RSV seasons, no statistically significant difference between maternal RSVpreF and infant nirsevimab in RSV infection or hospitalization was detected. Compared with no protection, both products were associated with fewer infections and hospitalizations. This study supports current US guidelines for either maternal RSVpreF or infant nirsevimab to protect all infants from RSV.