Background: A number of highly effective COVID-19 vaccines have been developed and approved for mass vaccination . We evaluated the impact of vaccination on COVID-19 outbreak and disease outcomes in Ontario, Canada .
Methods: We used an agent-based transmission model and parameterized it with COVID-19 characteristics, demographics of Ontario, and age-specific clinical outcomes . We implemented a two-dose vaccination program according to tested schedules in clinical trials for Pfizer-BioNTech and Moderna vaccines, prioritizing healthcare workers, individuals with comorbidities, and those aged 65 and older . Daily vaccination rate was parameterized based on vaccine administration data . Using estimates of vaccine efficacy, we projected the impact of vaccination on the overall attack rate, hospitalizations, and deaths . We further investigated the effect of increased daily contacts at different stages during vaccination campaigns on outbreak control .
Results: Maintaining non-pharmaceutical interventions (NPIs) with an average of 74% reduction in daily contacts, vaccination with Pfizer-BioNTech and Moderna vaccines was projected to reduce hospitalizations by 27.3% (95% CrI : 22.3% - 32.4 %) and 27.0% (95% CrI : 21.9% - 32.6 %), respectively, over a one-year time horizon . The largest benefits of vaccination were observed in preventing deaths with reductions of 31.5% (95% CrI : 22.5% - 39.7 %) and 31.9% (95% CrI : 22.0% - 41.4 %) for Pfizer-BioNTech and Moderna vaccines, respectively, compared to no vaccination . We found that an increase of only 10% in daily contacts at the end of lockdown, when vaccination coverage with only one dose was 6%, would trigger a surge in the outbreak . Early relaxation of population-wide measures could lead to a substantial increase in the number of infections, potentially reaching levels observed during the peak of the second wave in Ontario .
Conclusions: Vaccination can substantially mitigate ongoing COVID-19 outbreaks . Sustaining population-wide NPIs, to allow for a sufficient increase in population-level immunity through vaccination, is essential to prevent future outbreaks.