Neighborhood-level vaccine impact on COVID-19 infection and hospital admission in Quebec, Canada, during the Delta and early Omicron periods

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ObjectiveTo assess the impact of COVID-19 vaccination on COVID-19 infection and hospitalisation at the population-level, and to assess the indirect effects of vaccination in the province of Quebec, Canada. MethodsWe performed a time-stratified, neighborhood-level ecologic study. The exposure was neighborhood-level vaccination (primary series) coverage; outcomes were COVID-19 infection and hospitalisation rates. We used robust Poisson regression to estimate weekly relative rates of infection and hospitalisation versus vaccination. ResultsHigher vaccination coverage was associated with lower COVID-19 infection rates from July 18-December 4 for the year 2021 (Delta period) (RR≈0.46 [0.39; 0.54] – 0.94 [0.83; 1.05], 85–100% vs. 60–74% coverage). From December 5-December 25, this association reversed (RR≈1.28 [1.16; 1.41] – 1.41 [1.31; 1.52]), possibly due to the Omicron variant, social behaviors and accumulation of susceptibles in more vaccinated neighborhoods. Vaccine impact against hospitalisation was maintained throughout (RR≈0.43 [0.29; 0.65] – 0.88 [0.64; 1.22]). Vaccination provided substantial indirect protection (RR≈0.43 [0.34; 0.54] – 0.81 [0.65; 1.03]). ConclusionsThis study confirmed the protective impact of vaccination against severe disease regardless of variant, at the population level. Ecological analyses are a valuable strategy to evaluate vaccination programs. Population-level effects can have substantial effects and should be accounted for in public health and vaccination program planning.