Temporal trends in RSV-specific antibody seroprevalence among hosptalized children during the COVID-19 pandemic in Japan: a retrospective cross-sectional study

Respiratory syncytial virus (RSV) is a major cause of acute respiratory illness in young children. Given the transient nature of natural immunity, understanding age-specific seroprevalence is critical for optimising vaccine development and strategies. This study investigated the impact of the COVID-19 pandemic on RSV immunity in children under five (0–60 months), specifically aiming to determine the age with the lowest antibody prevalence and assess overall RSV seroprevalence during the pandemic years (2019–2021). Analysis of serum IgG concentrations revealed a significant decline in overall RSV IgG levels and seropositivity rates. Median IgG concentrations dropped from 912.94 mIU/mL in 2019 to 527.74 mIU/mL in 2021 (p < 0.001). A one-way ANOVA confirmed a significant effect of year on IgG levels (p < 0001), with post-hoc Tukey HSD tests showing significant differences between all annual pairs (2019 vs 2020: p < 0.0001; 2020 vs 2021: p = 0.0015). Age-stratified analysis consistently showed a U-shaped pattern in mean log IgG levels across all years: high in young infants (maternal antibodies), declining through early childhood, and increasing in older children. The IgG lowest point was delayed in 2021 (9–12 months) versus 2019 (5–8 months). Consistently lower IgG levels and seropositivity rates were observed in 2020 and 2021. Peak second quarter seropositivity dropped from 50.0–50.3% (2019–2020) to 25.0% (2021), and older children’s peak seropositivity in 2021 was 47.0% vs. 85.0% in 2019. These findings suggest that reduced RSV circulation during the COVID-19 pandemic may have yielded lower RSV-specific antibody levels among hospitalized children. While not representative of the general pediatric population, these results highlight the value of hospital-based serological monitoring to detect shifts in viral exposure and inform immunization strategies.