Riding high: seroprevalence of SARS-CoV-2 after 4 pandemic waves in Manitoba, Canada, April 2020-February 2022

Abstract

Background: Canada is emerging from the largest SARS-CoV-2 Omicron wave to date, with over 3.3 million confirmed cases. Unfortunately, PCR confirmed cases illuminate only a small portion of infections in the community and underestimate true disease burden. Population based seroprevalence studies, which measure antibody levels against a virus can more accurately estimate infection rates in the community and identify geographical and epidemiological trends to inform public health responses.

Methods: The Manitoba COVID-19 Seroprevalence (MCS) study is a population-based cross-sectional study to assess the prevalence of SARS-CoV-2 antibodies across the province. Residual convenience specimens (n = 14,901) were tested for anti-SARS-CoV-2 nucleocapsid and spike IgG antibodies from April 1, 2020 to February 31, 2022. We estimated the monthly and cumulative prevalence using an exponential decay model, accounting for population demographics, sensitivity/specificity, and antibody waning. This approach generated estimates of natural infection as well as total antibody including vaccine-induced immunity within the community.

Findings: After four waves of the pandemic, 60.1% (95%CI-56.6-63.7) of Manitobans have generated SARS-CoV-2 antibodies due to natural exposure independent of vaccination. Geographical analysis indicates a large portion of provincial prevalence stems from increased transmission in the Northern (92.3%) and Southern (71.8%) regional health authorities. Despite the high mortality rates reported by Manitoba, infection fatality ratios (IFR) peaked at 0.67% and declined to 0.20% following the Omicron wave, indicating parity with other national and international jurisdictions. Manitoba has achieved 93.4% (95%CI- 91.5-95.1) total antibody when including vaccination.

Interpretation: Our data shows that more than 3 in 5 Manitobans have been infected by SARS-CoV-2 after four waves of the pandemic. This study also identifies key geographical and age specific prevalence rates that have contributed greatly to the overall severity of the pandemic in Manitoba and will inform jurisdictions considering reduction of public health measures.

Keywords: SARS-CoV-2; Serology; Seroprevalence; Vaccination.

Fig. 1

Seroprevalence in Manitoba, Canada during 4 pandemic waves. SARS-CoV-2 PCR-positive reporting dates (grey) were plotted and then backdated by 6 days to estimate symptom onset dates (red). Lab validation data was used to extrapolate and plot the expected seroconversion waves (blue). Sampling months outlined in grey boxes with total antibody (solid purple) and natural infection (solid green) seroprevalence rates for Manitoba are depicted along with raw rates (dashed). Raw and corrected seroprevalence rates are also depicted by sex (b), age (c), and regional health authority (d). Winnipeg Regional Health Authority (WRHA); Northern Regional Health Authority (NRHA); Southern Health-Sante Sud (SHSS); Prairie Mountain Health (PMH); Interlake-Eastern Regional Health Authority (IERHA); Error bars depict 95% confidence intervals

Fig. 2

Infection fatality ratio estimates for Manitoba during 4 pandemic waves. Reported deaths (blue) and primary care home deaths (green) are illustrated along with the overall infection fatality ratio including (solid red) and excluding (dashed red) primary care home deaths. IFR estimates are also illustrated by sex (b), age group (c), and regional health authority (d). IFR rates for regional health authority are presented as crude rates. Winnipeg Regional Health Authority (WRHA); Northern Regional Health Authority (NRHA); Southern Health-Sante Sud (SHSS); Prairie Mountain Health (PMH); Interlake-Eastern Regional Health Authority (IERHA); Error bars depict 95% confidence intervals

Fig. 3

Geographical progression of seroprevalence in Manitoba, Canada and the greater Winnipeg area over 4 waves of the SARS-CoV-2 pandemic. Natural infections (red) and negative specimens (blue) are depicted for select months; April 2020 (a), May 2021 (b), and February 2022 (c)