Clinical Severity of Omicron SARS-CoV-2 Variant Relative to Delta in British Columbia, Canada: A Retrospective Analysis of Whole Genome Sequenced Cases

Background: In late 2021, the Omicron SARS-CoV-2 variant emerged and rapidly replaced Delta as the dominant variant globally. The increased transmissibility of the variant led to surges in case rates as well as increases in hospitalizations, however, the true severity of the variant remained unclear. We aimed to provide robust estimates of Omicron severity relative to Delta.Methods: This study was conducted using a retrospective cohort design with data from the British Columbia COVID-19 Cohort – a large provincial surveillance platform with linkage to administrative datasets. To capture the time of co-circulation with Omicron and Delta, December 2021 was chosen as the study period. We included individuals diagnosed with Omicron or Delta infection, as determined by whole genome sequencing (WGS). To assess the severity (hospitalization, ICU admission, length of stay) of Omicron, we conducted adjusted Cox proportional hazard models, weighted by inverse probability of treatment weights (IPTW).Findings: The cohort was composed of 13,128 individuals (7,729 Omicron and 5,399 Delta). There were 419 COVID-19 hospitalizations, with 118 (22%) among people diagnosed with Omicron (crude rate=1·5% Omicron, 5·6% Delta). In multivariable IPTW analysis, Omicron was associated with a 50% lower risk of hospitalization compared to Delta (aHR=0·50; 95%CI=0·43-0·59), a 73% lower risk of ICU admission (aHR=0·27; 95%CI=0·19-0·38), and a 5 days shorter hospital stay on average (ß=-5·03; 95% CI=-8·01 - -2·05).Interpretation: Our analysis supports findings from other studies demonstrating an association between Omicron and a lower risk of severe outcomes relative to Delta.Funding Information: Canadian Institutes for Health Research (Institute of Infection and Immunity). Declaration of Interests: MK has grants and contracts with AbCellera, Roche, Hologic, and Siemens, all of which unrelated to this study. NZJ participates in Abbvie Advisory Board meeting. All others authors declare no conflicts of interest.Ethics Approval Statement: This study was reviewed and approved by the Behavioural Research Ethics Board at the University of British Columbia (#H20-02097).