Evaluation of Respiratory Syncytial Virus Prefusion F IgG Antibodies in Japanese Mother–Infant Pairs Following Maternal Abrysvo Vaccination and in Infants Hospitalized with RSV Infection

Background/Objectives: The approval of Abrysvo, a maternal bivalent respiratory syncytial virus (RSV) prefusion F (pre-F) vaccine, marked a major milestone in pediatric prophylaxis. However, real-world serological data on transplacental antibody transfer in Asian populations remains limited. This brief report evaluated serum RSV pre-F immunoglobulin G (IgG) geometric mean titers (GMTs) across three distinct Japanese cohorts. Methods: This single-center, comparative study analyzed neonates born to Abrysvo-vaccinated mothers (n = 12), neonates born to unvaccinated mothers (n = 40), and infants aged <6 months hospitalized with laboratory-confirmed RSV infection (n = 18) using enzyme-linked immunosorbent assay (ELISA). Results: Neonates of vaccinated mothers exhibited significantly higher pre-F IgG GMTs (4.74; 95% CI: 4.30–5.18) than those of unvaccinated mothers (3.80; 95% CI: 3.50–4.14). Conversely, infants requiring hospitalization for RSV infection demonstrated critically low pre-F IgG GMTs (1.08; 95% CI: 0.65–2.15), revealing statistically significant differences across all three groups (p < 0.001). Conclusions: Maternal immunization with Abrysvo significantly enhances passive humoral immunity in Japanese neonates compared to natural background immunity. Given the severe antibody deficiency observed in hospitalized infants, expanding maternal vaccination coverage is a high-priority public health strategy to mitigate the pediatric RSV burden in the post-pandemic era.