Abstract
Background
A major goal of COVID-19 vaccination is to prevent severe outcomes (hospitalizations and deaths). We estimated the effectiveness of mRNA and ChAdOx1 COVID-19 vaccines against severe outcomes in four Canadian provinces between December 2020 and September 2021.
Methods
We conducted this multiprovincial retrospective test-negative study among community-dwelling adults aged ≥18 years in Ontario, Quebec, British Columbia, and Manitoba using linked provincial databases and a common study protocol. Multivariable logistic regression was used to estimate province-specific vaccine effectiveness against COVID-19 hospitalization and/or death. Estimates were pooled using random effects models.
Results
We included 2,508,296 tested subjects, with 31,776 COVID-19 hospitalizations and 5,842 deaths. Vaccine effectiveness was 83% after a first dose, and 98% after a second dose, against both hospitalization and death (separately). Against severe outcomes (hospitalization or death), effectiveness was 87% (95%CI: 71%–94%) ≥84 days after a first dose of mRNA vaccine, increasing to 98% (95%CI: 96%–99%) ≥112 days after a second dose. Vaccine effectiveness against severe outcomes for ChAdOx1 was 88% (95%CI: 75%–94%) ≥56 days after a first dose, increasing to 97% (95%CI: 91%–99%) ≥56 days after a second dose. Lower one-dose effectiveness was observed for adults aged ≥80 years and those with comorbidities, but effectiveness became comparable after a second dose. Two doses of vaccines provided very high protection for both homologous and heterologous schedules, and against Alpha, Gamma, and Delta variants.
Conclusions
Two doses of mRNA or ChAdOx1 vaccines provide excellent protection against severe outcomes of hospitalization and death.
Keywords: SARS-CoV-2, hospitalization, death, vaccine effectiveness, test-negative design, Canada
Contributor Information
Sharifa Nasreen, Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario, Canada; ICES, Toronto, Ontario, Canada.
Yossi Febriani, Centre Hospitalier Universitaire (CHU) de Québec-Université Laval Research Center, Quebec City, Quebec, Canada.
Héctor Alexander Velásquez García, British Columbia Centre for Disease Control, Vancouver, British Columbia, Canada; School of Population and Public Health, University of British Columbia, Vancouver, British Columbia, Canada.
Geng Zhang, Vaccine and Drug Evaluation Centre, Department of Community Health Sciences, University of Manitoba, Winnipeg, Manitoba, Canada.
Mina Tadrous, ICES, Toronto, Ontario, Canada; Women’s College Hospital, Toronto, Ontario, Canada; Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, Ontario, Canada.
Sarah A Buchan, Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario, Canada; ICES, Toronto, Ontario, Canada; Public Health Ontario, Toronto, Ontario, Canada; Centre for Vaccine Preventable Diseases, University of Toronto, Toronto, Ontario, Canada.
Christiaan H Righolt, Vaccine and Drug Evaluation Centre, Department of Community Health Sciences, University of Manitoba, Winnipeg, Manitoba, Canada.
Salaheddin M Mahmud, Vaccine and Drug Evaluation Centre, Department of Community Health Sciences, University of Manitoba, Winnipeg, Manitoba, Canada.
Naveed Zafar Janjua, British Columbia Centre for Disease Control, Vancouver, British Columbia, Canada; School of Population and Public Health, University of British Columbia, Vancouver, British Columbia, Canada; Centre for Health Evaluation and Outcome Sciences, St Paul’s Hospital, Vancouver, British Columbia, Canada.
Mel Krajden, British Columbia Centre for Disease Control, Vancouver, British Columbia, Canada; Department of Pathology and Laboratory Medicine, University of British Columbia, Vancouver, British Columbia, Canada.
Gaston De Serres, Centre Hospitalier Universitaire (CHU) de Québec-Université Laval Research Center, Quebec City, Quebec, Canada; Institut national de sante publique du Québec, Biological and Occupational Risks, Quebec City, Quebec, Canada; Laval University, Department of Social and Preventive Medicine, Faculty of Medicine, Quebec City, Quebec, Canada.
Jeffrey C Kwong, Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario, Canada; ICES, Toronto, Ontario, Canada; Public Health Ontario, Toronto, Ontario, Canada; Centre for Vaccine Preventable Diseases, University of Toronto, Toronto, Ontario, Canada; Department of Family and Community Medicine, University of Toronto, Toronto, Ontario, Canada; University Health Network, Toronto, Ontario, Canada.
Supplementary Material
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.