ABSTRACT
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), we conducted adjusted Cox proportional hazard models, weighted by inverse probability of treatment weights (IPTW), accounting for age, sex, underlying comorbidities, vaccination, sociodemographic status, and geographical variation.
Results
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 (aß=-5.03; 95% CI=-8.01, -2.05).
Conclusions
Our analysis supports findings from other studies demonstrating lower risk of severe outcomes in Omicron-infected individuals relative to Delta.
Keywords: SARS-CoV-2, COVID-19, Omicron, Delta, severity
Contributor Information
Sean P Harrigan, British Columbia Centre for Disease Control, British Columbia (BC), Canada; University of British Columbia Centre for Disease Control, BC, Canada.
James Wilton, British Columbia Centre for Disease Control, British Columbia (BC), Canada.
Mei Chong, British Columbia Centre for Disease Control, British Columbia (BC), Canada.
Younathan Abdia, British Columbia Centre for Disease Control, British Columbia (BC), Canada.
Hector Velasquez Garcia, British Columbia Centre for Disease Control, British Columbia (BC), Canada.
Caren Rose, British Columbia Centre for Disease Control, British Columbia (BC), Canada; University of British Columbia, School of Population and Public Health, BC, Canada.
Marsha Taylor, British Columbia Centre for Disease Control, British Columbia (BC), Canada.
Sharmistha Mishra, Department of Medicine, University of Toronto, Toronto, Canada; MAP Centre for Urban Health Solutions, Li Ka Shing Knowledge Institute, Unity Health Toronto, Toronto, Canada; Division of Epidemiology and Institute of Health Policy, Management and Evaluation, Dalla Lana School of Public Health, University of Toronto, Toronto, Canada; Institute of Medical Science, University of Toronto, Toronto, Canada.
Beate Sander, Toronto Health Economics and Technology Assessment (THETA) collaborative, Toronto General Hospital Research Institute, University Health Network, Toronto, Canada; Institute of Health Policy, Management and Evaluation (IHPME), Dalla Lana School of Public Health, University of Toronto, Toronto, Canada; Public Health Ontario Toronto, Canada; ICES, Toronto, Canada.
Linda Hoang, British Columbia Centre for Disease Control, British Columbia (BC), Canada; University of British Columbia, Pathology and Laboratory Medicine, BC, Canada.
John Tyson, British Columbia Centre for Disease Control, British Columbia (BC), Canada.
Mel Krajden, British Columbia Centre for Disease Control, British Columbia (BC), Canada; University of British Columbia, Pathology and Laboratory Medicine, BC, Canada.
Natalie Prystajecky, British Columbia Centre for Disease Control, British Columbia (BC), Canada; University of British Columbia, Pathology and Laboratory Medicine, BC, Canada.
Naveed Z Janjua, British Columbia Centre for Disease Control, British Columbia (BC), Canada; University of British Columbia, School of Population and Public Health, BC, Canada; Centre for Health Evaluation and Outcome Sciences, St. Paul's Hospital, Vancouver, BC, Canada.
Hind Sbihi, British Columbia Centre for Disease Control, British Columbia (BC), Canada; University of British Columbia, School of Population and Public Health, BC, Canada.
Supplementary Material
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