Longitudinal RSV Antibody Trends in Thai Children (0-8 Years): Association With Seasonal Outbreak Patterns and Potential Immunity Debt During the COVID-19 Pandemic.

Respiratory syncytial virus (RSV) remains a major cause of respiratory illness in young children, typically exhibiting seasonal transmission patterns. However, the COVID-19 pandemic disrupted these patterns, leading to atypical resurgences and raising concerns about the concept of "immunity debt." In tropical settings, the role of naturally acquired immunity in RSV transmission remains underexplored. This study investigates longitudinal trends in RSV-specific antibody levels among Thai children before, during, and after the COVID-19 pandemic. A total of 3219 serum samples, collected from mothers once at delivery and from children annually between 2015 and 2024, were tested for anti-RSV IgG using ELISA. Antibody levels aligned with annual RSV seasonality in Thailand, increasing shortly after epidemic peaks and declining during off-seasons. Seropositivity rose with age, reaching 97.5% by 96 months. Notably, children aged 60-72 months sampled during the pandemic exhibited reduced IgG levels, coinciding with public health restrictions. Geometric mean antibody levels declined from 100.10 RU/mL in 2019 to 89.03 in 2022, then significantly rebounded to 107.00 RU/mL in 2023 following the easing of restrictions and the atypical off-season resurgence. These findings support the immunity debt hypothesis and underscore the need for sustained serological surveillance and immunization strategies to mitigate future RSV outbreaks.