Direct and indirect effects of COVID-19 on infant bronchiolitis: a population-wide registry study from Northern Italy

BackgroundCOVID-19 pandemic modified trends of infant bronchiolitis, leading cause of emergency room (ER) attendance in infants in high income countries, primarily due to Respiratory Syncytial Virus. Understanding recent pandemic-related epidemiological shifts could inform affordable prevention efforts to improve sustainability and coverage of prophylactic treatments beyond maternal vaccines and infant monoclonal antibodies. MethodsWe estimated incidence rates (IR) of bronchiolitis ER presentations in 0-6month olds in 2012-2023 in a region-wide cohort of almost 1 million births using administrative health data from Lombardy, Italy. To assess potential contributors, we investigated direct and indirect COVID-19-related factors, including disease severity, shape of the epidemic peak, parental health seeking behaviour and vaccine coverage. We estimated COVID-19 maternal vaccination associations using mothers-infants data linkages and multivariable-adjusted logistic regression. FindingsCompared to pre-pandemic levels, IRs doubled in 2021-2023 (from 2 to [≥] 4/100 person-time). The surge could not be explained by changes in infant age at ER visit, clinical severity or in the shape of the epidemic peak. Adjusted models showed little support for a role of infant routine or maternal COVID-19 vaccination on bronchiolitis episodes (respectively hazard ratio 1.09, 95%CI: 0.98-1.22 and odds ratio 0.91, 95%CI: 0.76-1.09). InterpretationThe post-pandemic increase in infant bronchiolitis is likely driven by multiple epidemiological changes rather than a single factor. As maternal RSV vaccination and infant RSV monoclonal antibodies are introduced and scale up, identifying these mechanisms will be critical to maximise their public health impact, inform complementary prevention strategies, and improve preparedness for future RSV epidemics.