Background
Respiratory syncytial virus (RSV) is a leading cause of hospitalisation and mortality in young children globally. The social distancing measures implemented against COVID-19 in Lombardy (Italy) disrupted the typically seasonal RSV circulation during 2019–2021 and caused substantially more hospitalisations during 2021–2022. The primary aim of this study is to quantify the immunity gap — defined as the increased proportion of the population naïve to RSV infection — following the relaxation of COVID-19 restrictions in Lombardy, which has been hypothesised to be a potential cause of the increased RSV burden in 2021–2022.

Methods
We developed a catalytic model to reconstruct changes in the age-dependent susceptibility profile of the Lombardy population throughout the COVID-19 pandemic. The model is calibrated to routinely collected hospitalisation, syndromic, and virological surveillance data and tested for alternative assumptions on age-dependencies in the risk of RSV infection throughout the pandemic.

Findings
We estimate that the proportion of the Lombardy population naïve to RSV infection increased by 60·8% (95% CrI: 55·2–65·4%) during the COVID-19 pandemic: from 1·4% (95% CrI: 1·3–1·6%) in 2018–2019 to 2·3% (95% CrI: 2·2–2·5%) before the 2021–2022 season, corresponding to an immunity gap of 0·87% (95% CrI: 0·87–0.88%). We found evidence of heterogeneity in RSV transmission by age, suggesting that the COVID-19 restrictions had variable impact on the contact patterns and risk of RSV infection across ages.

Interpretation
We estimate a substantial increase in the population-level susceptibility to RSV in Lombardy during 2019–2021, which contributed to an increase in primary RSV infections in 2021–2022.