Projecting the impact of a two-dose COVID-19 vaccination campaign in Ontario, Canada

Background: Results of phase III vaccine clinical trials against COVID-19, although encouraging and well above initial expectations, have only reported on efficacy against disease and its severity. 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 derived from outbreak data. We implemented a two-dose vaccination program, prioritizing healthcare workers and high-risk individuals, with 40% vaccine coverage and vaccine efficacy of 95% against disease. Vaccines were distributed at a rate of 30 doses per day per 10,000 population with a 6-day schedule per week. We projected the impact of vaccination on attack rates, hospitalizations, and deaths. For scenario analyses, we varied the vaccine efficacy against infection, under the assumption of 5% pre-existing population immunity. Results: With no protection against infection, a two-dose vaccination campaign with a time interval of 21 days between doses reduced attack rate, hospitalizations, and deaths by 44.6% (95% CrI: 34.5% - 54.3%), 63.4% (95% CrI: 56.1% - 69.9%), and 70.0% (95% CrI: 62.6% - 75.8%), respectively. These reductions were improved with increased vaccine efficacy against infection, with similar estimated ranges in the corresponding scenarios with a 28-day time interval between vaccine doses. Conclusions: Vaccination can substantially mitigate ongoing COVID-19 outbreaks, even when vaccines offer limited protection against infection. This impact is founded upon a relatively strong vaccine efficacy against disease and severe outcomes.