Stringency of containment and closures on the growth of SARS-CoV-2 in Canada prior to accelerated vaccine roll-out

Background — Many studies have examined the effectiveness of non-pharmaceutical interventions (NPIs) on SARS-CoV-2 transmission worldwide. However, less attention has been devoted to understanding the limits of NPIs across the course of the pandemic and along a continuum of their stringency. In this study, we explore the relationship between the growth of SARS-CoV-2 cases and an NPI stringency index across Canada prior to accelerated vaccine roll-out.

Methods — We conducted an ecological time-series study of daily SARS-CoV-2 case growth in Canada from February 2020 to February 2021. Our outcome was a back-projected version of the daily growth ratio in a stringency period (i.e., a 10-point range of the stringency index) relative to the last day of the previous period. We examined the trends in case growth using a linear mixed effects model accounting for stringency period, province, and mobility in public domains.

Results — Case growth declined, rapidly, by 20–60% and began plateauing within the first two weeks of the first wave, irrespective of the starting values of the stringency index. When changes in stringency periods occurred, any decrease in case growth was not immediate and was faster in the first wave (by day nine) than in the second (by day 16). In the first wave, the largest decreasing trends from our mixed effects model occurred in both early and late stringency periods depending on the province, at a geometric mean index value of 30•1 out of 100. When compared to the first wave, the stringency periods in the second wave possessed little association with case growth.

Conclusions — The minimal association in the first wave, and the lack thereof in the second, is compatible with the hypothesis that NPIs do not, per se, lead to a decline in case growth. Instead, the correlations we observed might be better explained by a combination of underlying behaviours of the populations in each province and the natural dynamics of SARS-CoV-2. While there exist alternative explanations for the equivocal relationship between NPIs and case growth, the onus of providing evidence shifts to demonstrating how NPIs can consistently have flat association despite incrementally high stringency.