Relative Virulence of SARS-CoV-2 Among Vaccinated and Unvaccinated Individuals Hospitalized with SARS-CoV-2
Abstract
Background
The rapid development of safe and effective vaccines against the SARS-CoV-2 virus has been a singular scientific achievement. Confounding due to health seeking behaviours and differential testing by vaccination status may bias analyses towards an apparent increase in infection severity following vaccination. We sought to determine whether risks of intensive care unit (ICU) admission and death were diminished significantly by vaccination, even in individuals for whom vaccination failed to prevent hospitalization.
Methods
We used data from Ontario, Canada’s Case and Contact Management database, merged to a provincial vaccination dataset (COVaxON) to create a time-matched cohort of individuals who were hospitalized with SARS-CoV-2 infection. Each vaccinated individual was matched to up to five unvaccinated individuals based on test date of positive SARS-CoV-2 infection. Risk of ICU admission and death were evaluated using multivariable conditional logistic regression. Unmatched exploratory analyses were performed to identify sources of heterogeneity in vaccine effects.
Results
In 20,064 individuals (3,353 vaccinated and 16,711 unvaccinated) hospitalized with infection due to SARS-CoV-2 between January 1 st , 2021 and January 5 th , 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) for ICU admission per additional dose: 0.66, 95% CI 0.62 to 0.71; aOR for death per additional dose: 0.78, 95% CI 0.72 to 0.84). The reduction in risk was greater for ICU admission than for death (P for heterogeneity <0.05), but no significant differences in risk were seen based on infecting variant of concern (VOC).
Interpretation
We identified a decrease in the risk of ICU admission and death in vaccinated individuals compared to unvaccinated, time-matched controls, even when vaccines failed to prevent infection sufficiently severe to cause hospitalization. Even with diminished efficacy of vaccines against infection with novel VOCs, vaccines remain an important tool for reduction of ICU admission and mortality.
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