Ana Cecilia Ulloa, Sarah A. Buchan, Nick Daneman, Kevin A. Brown: Early estimates of SARS-CoV-2 Omicron variant severity based on a matched cohort study, Ontario [Preprint], Canada, in: medRxiv (January 2, 2022), online in: https://doi.org/10.1101/2021.12.24.21268382.
While it was evident in January 2022 that Omicron is rapidly replacing Delta, largely due to immune escape, it was less clear how the severity of Omicron compares to Delta. In Ontario, the authors sought to examine hospitalization and death associated with Omicron, as compared to cases infected with Delta. The authors conducted a matched cohort study, considering time to hospitalization or death as the outcome. Cases were matched on gender, age, vaccination status, health region and onset date. They identified 29,594 Omicron cases that met eligibility criteria, of which 11,622 could be matched with at least one Delta case (N=14,181). There were 59 (0.51%) hospitalizations and 3 (0.03%) deaths among matched Omicron cases, compared to 221 (1.6%) hospitalizations and 17 (0.12%) deaths among matched Delta cases. The risk of hospitalization or death was 65% lower (hazard ratio, HR=0.35, 95%CI: 0.26, 0.46) among Omicron cases compared to Delta cases, while risk of intensive care unit admission or death was 83% lower (HR=0.17, 95%CI: 0.08, 0.37). While severity is likely to be reduced, the absolute number of hospitalizations and impact on the healthcare system may nevertheless be significant due to the increased transmissibility of Omicron.
Competing Interest Statement
The authors have declared no competing interest.
This study did not receive any external funding.