LESSONS FROM THE COVID-19 THIRD WAVE IN CANADA: THE IMPACT OF VARIANTS OF CONCERN AND SHIFTING DEMOGRAPHICS

McAlister, F. A.; Nabipoor, M.; Chu, A.; Lee, D.; Saxinger, L.; Bakal, J.

Importance: With the emergence of more transmissible SARSCoV2 variants of concern (VOC), there is an urgent need for evidence about disease severity and the health care impacts of VOC in North America, particularly since a substantial proportion of the population have declined vaccination thus far. Objective: To examine 30day outcomes in Canadians infected with SARSCoV2 in the first year of the pandemic and to compare event rates in those with VOC versus wild type infection. Design: Retrospective cohort study using linked healthcare administrative datasets. Setting: Alberta and Ontario, the two Canadian provinces that experienced the largest third wave in the spring of 2021. Participants: All individuals with a positive SARSCoV2 reverse transcriptase polymerase chain reaction swab from March 1, 2020 until March 31, 2021, with genomic confirmation of VOC screen positive tests during February and March 2021 (wave 3). Exposure of Interest: VOC versus wild type SARSCoV2 Main Outcomes and Measures: All-cause hospitalizations or death within 30 days after a positive SARSCoV2 swab. Results: Compared to the 372,741 individuals with SARSCoV2 infection between March 2020 and January 2021 (waves 1 and 2 in Canada), there was a shift in transmission towards younger patients in the 104,232 COVID19 cases identified in wave 3. As a result, although third wave patients were more likely to be hospitalized (aOR 1.34 [1.29 to 1.39] in Ontario and aOR 1.53 [95%CI 1.41 to 1.65] in Alberta), they had shorter lengths of stay (median 5 vs. 7 days, p<0.001) and were less likely to die within 30 days (aOR 0.66 [0.60 to 0.71] in Ontario and aOR 0.74 [0.62 to 0.89] in Alberta). However, within the third wave, patients infected with VOC (91% Alpha) exhibited higher risks of death (aOR 1.52 [1.27 to 1.81] in Ontario and aOR 1.67 [1.13 to 2.48] in Alberta) and hospitalization (aOR 1.57 [1.47 to 1.69] in Ontario and aOR 1.88 [1.74 to 2.02] in Alberta) than those with wild-type SARSCoV2 infections during the same timeframe. Conclusions and Relevance: On a population basis, the shift towards younger age groups as the COVID19 pandemic has evolved translates into more hospitalizations but shorter lengths of stay and lower mortality risk than seen in the first 10 months of the pandemic in Canada. However, on an individual basis, infection with a VOC is associated with a higher risk of hospitalization or death than the original wild type SARSCoV2; this is important information to address vaccine hesitancy given the increasing frequency of VOC infections now.