Epidemiology and severity of pediatric influenza hospitalizations before and during the COVID-19 pandemic: a surveillance study of the Canadian immunization monitoring program (IMPACT), 2012–2023

Background: The COVID-19 pandemic disrupted respiratory virus circulation, including influenza. We described trends in seasonality, demographics, and severity of pediatric influenza hospitalizations in Canada before and during the pandemic. Methods: We conducted national prospective surveillance through the Canadian Immunization Monitoring Program, ACTive (IMPACT), across 12 tertiary-care pediatric centers. Children <17 years hospitalized with laboratory-confirmed influenza between August 2012-August 2023 were included. Periods were categorized as pre-pandemic (2012-March 2020), and pandemic (2021-2022, 2022-2023). Severe disease was defined as intensive care unit (ICU) admission, mechanical ventilation, extracorporeal membrane oxygenation, and/or death. Adjusted odds ratios (aOR) were estimated using multivariable logistic regression. Findings: Among 10,091 hospitalizations, 7938 occurred pre-pandemic (annual average = 955), 335 in 2021-2022, and 1810 in 2022-2023. The median age was 3.0 (IQR 1.2-6.4) in pre-pandemic, 3.3 (IQR 1.4-7.3) in 2021-2022, and 3.8 years (IQR 1.7-7.2, p < 0.001) in 2022-2023. ICU admissions declined in 2021-2022 (11.3%) and rose in 2022-2023 (15.9%), though remaining below pre-pandemic levels (17.3%). Mortality was stable across periods (0.6%). The odds of severe disease were lower in 2021-2022 (aOR 0.63, 95% CI 0.44-0.89) and 2022-2023 (aOR 0.85, 95% CI 0.74-0.98) compared to pre-pandemic. Interpretation: Pediatric influenza hospitalizations nearly doubled in 2022-2023, with a modest shift in age distribution toward older children. Although severity indicators rose compared to 2021-2022, they remained slightly reduced compared to pre-pandemic seasons, suggesting suggest a partial return toward pre-pandemic epidemiologic patterns. Funding: IMPACT is managed by the Canadian Paediatric Society and conducted on behalf of the Public Health Agency of Canada.