Counterfactuals of effects of vaccination and public health measures on COVID-19 cases in Canada: What could have happened?

Abstract

This study illustrates what may have happened, in terms of coronavirus disease 2019 (COVID-19) infections, hospitalizations and deaths in Canada, had public health measures not been used to control the COVID-19 epidemic, and had restrictions been lifted with low levels of vaccination, or no vaccination, of the Canadian population. The timeline of the epidemic in Canada, and the public health interventions used to control the epidemic, are reviewed. Comparisons against outcomes in other countries and counterfactual modelling illustrate the relative success of control of the epidemic in Canada. Together, these observations show that without the use of restrictive measures and without high levels of vaccination, Canada could have experienced substantially higher numbers of infections and hospitalizations and almost a million deaths.

Characterizing responsiveness to the COVID-19 pandemic in the United States and Canada using mobility data

“…The strongest measures of responsiveness were seen in Canada’s Atlantic provinces, which adopted an elimination strategy during the study period. 21 Atlantic Canada saw much lower maximum COVID-19 case rates than the other Canadian provinces (figure S4) and recorded extremely low case rates throughout much of the study period. Atlantic Canada achieved high responsiveness by implementing strong public health measures following upticks in COVID-19 incidence that would be considered unremarkable in any other jurisdiction, in addition to adopting measures to prevent importation.…”

“…Atlantic Canada achieved high responsiveness by implementing strong public health measures following upticks in COVID-19 incidence that would be considered unremarkable in any other jurisdiction, in addition to adopting measures to prevent importation. 21…”

Which Curve Are We Flattening? The Disproportionate Impact of COVID-19 Among Economically Marginalized Communities in Ontario, Canada, Was Unchanged From Wild-Type to Omicron

“…It has been well-established in Canada and across countries [ 9 , 10 , 13 ] that differential exposure risks to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and subsequent mortality, were amplified at the intersections of household crowding and size [ 11 , 14 ], workplace exposures [ 15 , 16 ], and systemic barriers to prevention and care (including access to therapeutics; see [ 17–19 ]). Our findings should not be interpreted as though the public health measures, strategies, and interventions did not work, as evidence points to large, overall prevention in COVID-19 deaths [ 20 ]. Evaluation of temporary income support and eviction moratoria have been known to be associated with improved outcomes with respiratory viruses [ 21 ], with evidence of impact in the context of COVID-19 [ 22 , 23 ].…”

Impact of Immune Evasion, Waning and Boosting on Dynamics of Population Mixing Between a Vaccinated Majority and Unvaccinated Minority

“…The emergence of the Omicron variants of concern in late 2021 significantly diminished the efficacy of vaccines against infection with SARS-CoV-2, though there is a broad range of estimates respecting what initial protective efficacy might be. Estimates of 40-64% are most plausible (6)(7)(8); we conservatively used 40% in our base case, and varied initial efficacy across a range of 20-80% in sensitivity analyses. Initial immunity after infection was assumed perfect, with all infected people transiting to an immune state upon recovery from infection.…”

“…Most criticism focussed on the diminished vaccine efficacy associated with emergence of the Omicron variant, the fact that we had assumed durable immunity from vaccination in our published model, and the notion that giving unvaccinated people a "head start" of only 20% baseline immunity was insufficient. Evolving information on vaccine efficacy (6)(7)(8), durability of immune protection provided by vaccination and/or infection (9)(10)(11)(12)(13), impacts of vaccination and/or prior infection on infectivity among people with subsequent infection (14,15), and the availability and effects of booster doses (16,17), led us to update this earlier work. Our objectives were to evaluate whether the changing understanding of the attributes of SARS-CoV-2 vaccines and variants, and the availability of booster vaccination would result in a qualitative change in our earlier findings in projections using longer time horizons.…”