Background. The COVID-19 pandemic impacted RSV epidemiology. We describe Canadian pediatric tertiary care RSV hospitalizations in 2022-2023 and assess pandemic-related changes. Methods. Active surveillance of hospitalized children aged 0-16 years was conducted at 13 Immunization Monitoring Program, Active (IMPACT) centres. RSV hospitalizations in 2022-2023 were compared to those in the pre-pandemic period (2017-2018 through 2019-2020). Province-specific and age-stratified proportions of all-cause hospitalizations with RSV detection and age-stratified proportions of RSV-associated intensive care unit (ICU) admission were calculated. Changes in seasonality were assessed using Seasonal Autoregressive Integrated Moving Average (SARIMA) modelling. Results. In 2022-2023, there were 5362 RSV-associated hospitalizations including 1260 (23.5%) ICU admissions, both more than double pre-pandemic yearly averages. Overall, median (IQR) age increased from 6 months (1-20) to 9 months (2-27; P<0.001). The proportion of RSV hospitalizations among all-cause hospitalizations increased by 3.5 percentage points (95% CI 3.3-3.7 percentage points), to 6.8% (95% CI 6.6%-7.0%). While 41.5% of RSV hospitalizations were among children <6 months old, they accounted for 62% of ICU admissions. Overall, ICU proportion remained constant; however, odds of ICU admission among infants <6 months old increased (adjusted OR 1.35, 95% CI 1.2-1.52) compared to the pre-pandemic period. National weekly incidence in 2022-2023 peaked earlier, higher and persisted longer than expected by SARIMA. Interpretations. In 2022-2023, the number of RSV hospitalizations and ICU admissions increased dramatically in Canadian pediatric hospitals. Despite an older age distribution, the greatest burden remained in children <6 months old. RSV immunization strategies for young infants will likely have substantial potential public health impact.

Dr. Papenburg reports grants from MedImmune and Merck to his institution, and personal fees from Enanta, all outside the submitted work. Dr. Sadarangani is supported via a salary award from the BC Children's Hospital Foundation; in the last 3 years, he has been an investigator on projects funded by GlaxoSmithKline, Merck, Moderna, Pfizer, and Sanofi-Pasteur with all funds paid to his institute, and he has not received any personal payments. Dr. Halperin has served on ad hoc advisory boards and has been an investigator on projects funded by Sanofi-Pasteur, GlaxoSmithKline, Merck, Pfizer, VBI, CanSino, Moderna, and Seqirus. Dr. Morris has served on ad-hoc advisory boards for Pfizer, Sanofi Pasteur, GlaxoSmithKline, and Merck and has received speaker fees from GlaxoSmithKline, Sanofi Pasteur, and Pfizer, all unrelated to this study. Dr. Pernica reports receiving funding (all to McMaster) in grants from AstraZeneca and Merck and in-kind (diagnostics and consumables) in grants from bioMerieux and Abbott. Dr. Langley has been an investigator on projects funded by GlaxoSmithKline, Merck, Moderna, Pfizer, Sanofi-Pasteur, Seqirus, Symvivo, Entos, VBI Vaccines and CanSino with all funds paid to her employer, Dalhousie University and he has not received any personal payments. Dr. Langley holds the CIHR-GSK Chair in Pediatric Vaccinology at Dalhousie University. Dr Robinson has received honoraria for presentations from the Alberta Pharmacists' Association and the Canadian Paediatric Society and received payment for peer review from Elsevier. Dr. Kazmi has received honoraria for ad-hoc advisory boards for Takeda Canada and Bavarian Nordic, all unrelated to this study. The other authors have no conflicts of interest to disclose.

This surveillance activity is conducted as part of the Canadian Immunization Monitoring Program Active (IMPACT), a national surveillance initiative managed by the Canadian Pediatric Society (CPS) and conducted by the IMPACT network of pediatric investigators on behalf of the Public Health Agency of Canada's (PHAC's) Centre for Immunization and Respiratory Infectious Diseases. Funding for RSV surveillance was provided by PHAC. Participation by Aariana Lopes was supported by Mitacs through the Mitacs Accelerate program.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Ethics committee/IRB of Alberta Children's Hospital gave ethical approval for this work. Ethics committee/IRB of B.C. Children's Hospital gave ethical approval for this work. Ethics committee/IRB of Le Centre Mere-Enfant de Quebec City gave ethical approval for this work. Ethics committee/IRB of Children's Hospital of Eastern Ontario gave ethical approval for this work. Ethics committee/IRB of CHU-Sainte-Justine Montreal gave ethical approval for this work. Ethics committee/IRB of IWK Health Centre gave ethical approval for this work. Ethics committee/IRB of Eastern Health Janeway Child Health and Rehabilitation Centre gave ethical approval for this work. Ethics committee/IRB of The Montreal Children's Hospital gave ethical approval for this work. Ethics committee/IRB of Jim Pattison Children's Hospital gave ethical approval for this work. Ethics committee/IRB of Stollery Children's Hospital gave ethical approval for this work. Ethics committee/IRB of The Hospital for Sick Children gave ethical approval for this work. Ethics committee/IRB of Children's Hospital, Winnipeg, MB gave ethical approval for this work. Ethics committee/IRB of McMaster's Children Hospital gave ethical approval for this work.

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