Abstract
Respiratory viruses are a global health burden. While influenza has been a large focus of surveillance, non-influenza viruses remain understudied in Canada, leaving gaps in understanding their seasonal patterns and epidemiology. The coronavirus disease 2019 pandemic further disrupted typical virus seasonality, highlighting the urgent need for expanded surveillance to track evolving trends. A multicenter retrospective analysis of molecular testing data for 15 respiratory viruses was conducted on tests performed over three consecutive respiratory virus seasons. Hospital-level variability was assessed by calculating deviations of each hospital's peak week from the seasonal median peak week, summarized across years for each virus. We analyzed the timing of the weekly peak in test positivity for each virus. Each respiratory virus showed distinct yearly peak weeks, with influenza A virus, respiratory syncytial virus (RSV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) reaching the highest positivity rates (11.6%, 13%, and 11.7%, respectively). Significant shifts in peak timing were observed for adenovirus, SARS-CoV-2, influenza A virus, parainfluenza virus, and RSV across the 2022-2025 seasons, with adenovirus and SARS-CoV-2 peaking much earlier in 2024-2025 than in previous years. Influenza A virus peaked later, and RSV also peaked later only in the 2024-2025 season compared to 2023-2024. Overall, our results highlight shifts in respiratory virus transmission dynamics across Ontario hospitals. Ongoing surveillance will support targeted public health interventions and strategies for respiratory virus surges.