Understanding RSV Resurgence Following COVID-19 in Ontario, Canada: Evaluating the Roles of Contact Patterns and Maternal Immunity
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Background
Respiratory syncytial virus (RSV) remains a leading cause of hospitalization in infancy, with severe outcomes influenced by both contact patterns and passive immunity. Non- pharmaceutical interventions (NPIs) during the COVID-19 pandemic suppressed RSV circulation and reduced opportunities for maternal immune boosting, potentially altering protection among newborns. We evaluated whether incorporating time-varying maternal immunity improves the ability of an age-structured transmission model to predict post-pandemic RSV hospitalization patterns in infants.
Methods
We analyzed population-based RSV hospitalizations among Ontario (Canada) infants (<1 year) from July 2, 2017 to June 25, 2024, using linked administrative databases. A deterministic compartmental model across seven age classes was calibrated against pre-pandemic data using Latin Hypercube Sampling. We compared a model incorporating time-varying contact rates alone against a specification that additionally included time-varying maternal immunity.
Results
Both specifications accurately reproduced pre-pandemic seasonality and macro-level post-pandemic resurgence features. The constant maternal immunity model showed slightly better accuracy in capturing the 2021/22 peak compared to the time-varying maternal immunity specification. However, both qualitatively captured the continued near-absence of RSV and the observed peak was captured within the 95% credible intervals. While both models precisely captured the timing and overwhelming surge of admissions that occurred in 2022/23, they failed to capture the premature peak timing and magnitude in 2023/24.
Conclusions
Incorporating time-varying maternal immunity did not improve model accuracy post- pandemic. While maternal protection is essential for evaluating infant immunizations, population- level contact shifts primarily shaped post-pandemic RSV seasonality, indicating that models must account for these mechanisms of RSV transmission dynamics.
Summary
Using population-based Ontario hospitalization data, we evaluated whether incorporating time- varying maternal immunity improves an age-structured transmission model’s ability to reproduce infant RSV disease patterns. While maternal immunity did impact post-pandemic RSV resurgences, we found that population-level contact pattern shifts were primarily responsible.
