Changing Patterns of SARS-CoV-2 Seroprevalence among Canadian Blood Donors during the Vaccine Era

Abstract

We monitored the seroprevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid (anti-N; proxy of natural infection) and spike protein (anti-S; proxy for humoral immunity) antibodies in blood donors across Canada from January to November 2021. The first and second doses of vaccine were deployed over this time. Anti-N seroprevalence remained low overall (about 5% or lower) from January to November but was higher in racialized groups, younger age groups, and those living in materially deprived neighborhoods. Anti-S seroprevalence corresponded with the roll out of vaccines across the country, increasing in April in older donors and then progressively to younger age groups consistent with vaccination policies targeting oldest to youngest. By November, close to 100% of blood donors were positive for anti-S. Anti-S concentrations peaked by July and began waning by September to November particularly in older donors. These data have informed national and provincial public health policy in Canada throughout vaccination rollout. IMPORTANCE Throughout the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, our blood donor seroprevalence study has informed Canadian public health policy at national and provincial levels. We describe the only continuously running national seroprevalence study in Canada, which spans the full length of the pandemic and per capita is one of the largest programs in the world. The benefit of seroprevalence studies is that they identify a broad range of asymptomatic and symptomatic infection histories that may not be identified with active SARS-CoV-2 nucleic acid testing programs or when case definitions change. As vaccination was deployed in Canada, we estimated the proportion of donors with vaccine-related antibodies and developed population-level estimates of SARS-CoV-2 spike antibody concentrations. Monthly predictive mathematical models and our results engaged public health organizations in new ways. In the future, we intend to continue to expand on these interactions with provincial and national public health teams.

Keywords: Canada; SARS-CoV-2; blood donors; seroprevalence.

FIG 1

Number of retention samples tested by month from January to November 2021. A larger sample was tested in January, however, in February, samples were not tested. From March onward, samples from approximately the last 2 weeks of every month were tested; Jan, January; Aug, August; Sept, September; Oct, October; Nov, November.

FIG 2

Temporal trends of nucleocapsid antibody seroprevalence (with 95% confidence intervals) from January to November 2021 by sex, age group, ethnicity, and region.

FIG 3

Temporal trends of spike antibody seroprevalence (with 95% confidence intervals) from January to November 2021 by sex, age group, ethnicity, and region.

FIG 4

Overall temporal trends and distributions of spike antibody concentration (U/mL) by month from January to November 2021 (all values >2,500 U/mL classified as 2,500 U/mL; the white circle represents the median, and the thick bar represents the interquartile range [IQR]).

FIG 5

Distributions of spike antibody concentration (U/mL) by age group in spike antibody-positive donations in September (red), October (blue), and November (maroon) (dilution of 1:400 allows detection of up to 100,000 U/mL; the white circle represents the median, and the thick bar represents the interquartile range [IQR]).