Determinants of RSV epidemiology following suppression through pandemic contact restrictions

Abstract

Introduction COVID-19 related non-pharmaceutical interventions (NPIs) led to a suppression of RSV circulation in winter 2020/21 throughout Europe and an off-season resurgence in Summer 2021 in several European countries. We explore how such temporary interruption may shape future RSV epidemiology and what factors drive the associated uncertainty. Methods We developed an age-structured dynamic transmission model to simulate pre-pandemic RSV infections and hospitalisations. We sampled parameters governing RSV seasonality, immunity acquisition and duration of post-infection immunity and retained those simulations that qualitatively fit the UK's pre-pandemic epidemiology. From Spring 2020 to Summer 2021 we assumed a 50% reduced contact frequency, returning to pre-pandemic levels from mid-May 2021. We simulated transmission forwards until 2023 and evaluated the impact of the sampled parameters on the projected trajectories of RSV hospitalisations. Results Following a lifting of contact restrictions in summer 2021 the model replicated an out-of-season resurgence of RSV. If unmitigated, paediatric RSV hospitalisation incidence in the 2021/22 season was projected to increase by 32% to 67% compared to pre-pandemic levels. The size of the increase depended most on whether infection risk was primarily determined by immunity acquired from previous exposure or general immune maturation. While infants were less affected, the increase in seasonal hospitalisation incidence exceeded 100% in 1-2 year old children and 275% in 2-5 year old children, respectively, in some simulations where immunity from previous exposure dominated. Consequently, the average age of a case increased by 1 to 5 months, most markedly if there was strong immunity acquisition from previous exposure. If immunity to infection was largely determined by age rather than previous exposure, the 2021/22 season started earlier and lasted longer but with a peak incidence lower or similar to pre-pandemic levels. For subsequent seasons, simulations suggested a quick return to pre-pandemic epidemiology, with some slight oscillating behaviour possible depending on the strength of post-exposure immunity. Conclusion COVID-19 mitigation measures stopped RSV circulation in the 2020/21 season and generated immunity debt that will likely lead to a temporary increase in RSV burden in the season following the lifting of restrictions, particularly in 1 to 5 year old children. A more accurate understanding of immunity drivers for RSV is needed to better predict the size of such an increase and plan a potential expansion of pharmaceutical and non-pharmaceutical mitigation measures.

Year-round RSV Transmission in the Netherlands Following the COVID-19 Pandemic - A Prospective Nationwide Observational and Modeling Study

“…The Netherlands exhibited a unique pattern of RSV re-emergence during the COVID-19 pandemic characterized by a high summer peak in 2021 followed by a prolonged period of continuous transmission at mid- to high-level RSV activity. Our model simulations confirm that population “immunity debt” can explain the large RSV summer outbreak and the following “endemic phase” ( Supplementary Figure 5 ) [4, 7-9]. Additionally, NPIs were gradually re-implemented between mid-November 2021 and end-of-January 2022.…”

“…These two countries returned to a winter epidemic in 2021 with low RSV activity during the summer of 2022. NPIs strictness and virus importation from external sources could explain these observed variations in RSV seasonality as these factors shape the level of population immunity debt [6, 9] ( Supplementary Figure 4C ). Furthermore, differences in surveillance and reporting strategies may explain the observed difference.…”

Comparative analysis of pediatric Respiratory Syncytial Virus epidemiology and clinical severity before and during the COVID-19 pandemic in British Columbia, Canada

“…Modelling studies had predicted a resurgence of severe RSV cases (23–25). In contrast, our surveillance data showed increased RSV cases without increased severity.…”

Optimal Respiratory Syncytial Virus intervention programmes using Nirsevimab in England and Wales

“…The SARS-CoV-2 pandemic has caused seasonal shifts in RSV seasonality in England and Wales, and it remains unclear if a consistent seasonal peak will return to pre-pandemic level. [18,19] If there is a substantial change in RSV incidence and seasonality over the coming year, then limited inferences can be made from the results of this study on the impact and cost-effectiveness of Nirsevimab.…”

“…The seasonality of RSV in England and Wales has been largely unpredictable between 2020 and at the time of writing due to restrictions on social mixing due to the COVID-19 pandemic. [18,19] We make the simplifying assumption in this model that once Nirsevimab is licensed the observed seasonality prior to 2020 will return.…”