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Background: There have been multiple waves in the COVID-19 pandemic in many countries. We sought to compare mortality and respiratory, cardiovascular and renal dysfunction between waves in 3 Canadian provinces., Methods: We conducted a substudy of the ARBs CORONA I study, a multicentre Canadian pragmatic observational cohort study that examined the association of pre-existing use of angiotensin receptor blockers with outcomes in adults admitted to hospital with acute COVID-19 up to April 2021 from 9 community and teaching hospitals in 3 Canadian provinces (British Columbia, Ontario and Quebec). We excluded emergency department admissions without hospital admission, readmissions and admissions for another reason. We used logistic and 0-1-inflated β regression models to compare 28-day and in-hospital mortality, and the use of invasive mechanical ventilation, vasopressors and renal replacement therapy (RRT) between the first 3 waves of the COVID-19 pandemic in these provinces., Results: A total of 520, 572 and 245 patients in waves 1, 2 and 3, respectively, were included. Patients in wave 3 were on average younger and had fewer comorbidities than those in waves 1 and 2. The unadjusted 28-day mortality rate was significantly lower in wave 3 (7.8%) than in wave 1 (18.3%) (odds ratio [OR] 0.43, 95% confidence interval [CI] 0.24-0.78) and wave 2 (16.3%) (OR 0.46, 95% CI 0.27-0.79). After adjustment for differences in baseline characteristics, the difference in 28-day mortality remained significant (adjusted OR wave 3 v. wave 1: 0.46, 95% CI 0.26-0.81; wave 3 v. wave 2: 0.52, 95% CI 0.29-0.91). In-hospital mortality findings were similar. Use of invasive mechanical ventilation or vasopressors was less common in waves 2 and 3 than in wave 1, and use of RRT was less common in wave 3 than in wave 1., Interpretation: Severity of illness decreased (lower mortality and less use of organ support) across waves among patients admitted to hospital with acute COVID-19, possibly owing to changes in patient demographic characteristics and management, such as increased use of dexamethasone. Continued application of proven therapies may further improve outcomes., Study Registration: ClinicalTrials.gov, no. NCT04510623., Competing Interests: Competing interests: James Russell reports an investigator-initiated grant from Grifols that was provided to and administered by the University of British Columbia (UBC). He reports patents owned by UBC that are related to the use of PCSK9 inhibitor(s) in sepsis and the use of vasopressin in septic shock, and a patent owned by Ferring Pharmaceuticals for use of selepressin in septic shock. He is an inventor on these patents. He was a founder, director and shareholder in Cyon Therapeutics and is a shareholder in Molecular You. He reports consulting fees from SIB Therapeutics (developing a sepsis drug) and Ferring Pharmaceuticals (manufactures vasopressin and developing selepressin). He is no longer actively consulting for any industry. He was a nonfunded science advisor and member of the Government of Canada COVID-19 Therapeutics Task Force (June 2020–December 2021) and a nonfunded member of the data and safety monitoring board of a trial of plasma in COVID-19 (PassITON) (2020–2021) sponsored by the National Institutes of Health. No other competing interests were declared., (© 2022 CMA Impact Inc. or its licensors.)