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. 2023 Feb 8;76(3):e409-e415.
doi: 10.1093/cid/ciac412.

Relative Virulence of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Among Vaccinated and Unvaccinated Individuals Hospitalized With SARS-CoV-2

Alicia A Grima  1 , Kiera R Murison  1 , Alison E Simmons  1 , Ashleigh R Tuite  1 , David N Fisman  1
Affiliations

Relative Virulence of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Among Vaccinated and Unvaccinated Individuals Hospitalized With SARS-CoV-2

Alicia A Grima et al. Clin Infect Dis. .

Abstract

Background: The rapid development of safe and effective vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been a singular scientific achievement. Confounding due to health-seeking behaviors, circulating variants, and differential testing by vaccination status may bias analyses toward an apparent increase in infection severity following vaccination.

Methods: We used data from the Ontario, Canada, Case and Contact Management Database and a provincial vaccination dataset (COVaxON) to create a time-matched cohort of individuals who were hospitalized with SARS-CoV-2 infection. Vaccinated individuals were matched to up to 5 unvaccinated individuals based on test date. Risk of intensive care unit (ICU) admission and death were evaluated using conditional logistic regression.

Results: In 20 064 individuals (3353 vaccinated and 16 711 unvaccinated) hospitalized with infection due to SARS-CoV-2 between 1 January 2021 and 5 January 2022, vaccination with 1, 2, or 3 doses significantly reduced the risk of ICU admission and death. An inverse dose-response relationship was observed between vaccine doses received and both outcomes (adjusted odds ratio [aOR] per additional dose for ICU admission, 0.66; 95% confidence interval [CI], .62 to .71; aOR for death, 0.78; 95% CI, .72 to .84).

Conclusions: We identified decreased virulence of SARS-CoV-2 infections in vaccinated individuals, even when vaccines failed to prevent infection sufficiently severe to cause hospitalization. Even with diminished efficacy of vaccines against infection with novel variants of concern, vaccines remain an important tool for reduction of ICU admission and mortality.

Keywords: SARS-CoV-2; epidemiology; outcomes; pandemic; vaccination.

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Conflict of interest statement

Potential conflicts of interest. D. N. F. has served on advisory boards related to influenza and severe acute respiratory syndrome coronavirus 2 vaccines for Seqirus, Pfizer, AstraZeneca, and Sanofi-Pasteur Vaccines and has served as a legal expert on issues related to coronavirus disease 2019 epidemiology for the Elementary Teachers Federation of Ontario and the Registered Nurses Association of Ontario. All other authors report no potential conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

Figures

Figure 1.
Figure 1.
Flow diagram for the creation of the matched cohort. Abbreviations: CCM, Case and Contact Management System (Ontario’s line list database); COVaxON, Ontario provincial vaccination database.
Figure 2.
Figure 2.
Forest plot to evaluate heterogeneity between estimates by infecting variant and outcome. The analysis is stratified by outcome, with results for death in the upper rows and ICU admission below. Abbreviations: CI, confidence interval; ICU, intensive care unit; IV, contribution of within-stratum variance to overall variance; n501, N501Y-positive variant; VOC, variant of concern.
See this image and copyright information in PMC

References

    1. Johns Hopkins University Coronavirus Resource Center. COVID-19 Dashboard by the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University (JHU). Available at: https://coronavirus.jhu.edu/map.html. Last accessed 27 December 2021.
    1. Kreier F. “Unprecedented achievement”: who received the first billion COVID vaccinations? Nature 2021. doi:10.1038/d41586-021-01136-2. - DOI - PubMed
    1. Mesle MM, Brown J, Mook P, et al. . Estimated number of deaths directly averted in people 60 years and older as a result of COVID-19 vaccination in the WHO European Region, December 2020 to November 2021. Euro Surveill 2021; 26:2101021. - PMC - PubMed
    1. Gupta S, Cantor J, Simon KI, Bento AI, Wing C, Whaley CM. Vaccinations against COVID-19 may have averted up to 140,000 deaths in the United States. Health Aff (Millwood) 2021; 40:1465–72. - PMC - PubMed
    1. Hogan AB, Wu SL, Doohan P, et al. . Report 48—The value of vaccine booster doses to mitigate the global impact of the Omicron SARS-CoV-2 variant. 2021. Available at: http://www.imperial.ac.uk/mrc-global-infectious-disease-analysis/covid-1.... Last accessed 17 December 2021.

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