This is a summary, written by members of the CITF Secretariat, of:

Kanji JN, Nguyen LT, Plitt SS, Charlton CL, Fenton J, Braun S, Marohn C, Lau C, Svenson LW, Hinshaw D, Lutsiak C, Zelyas N, Mengel M, Tipples G. Seropositivity to SARS-CoV-2 in Alberta, Canada in a post-vaccination period (March 2021–July 2021). Infectious Diseases. 2022 May 31. DOI: 10.1080/23744235.2022.2080250

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

The proportion of individuals who tested positive for antibodies against the receptor binding domain (RBD), acquired from infection and/or vaccination, rose from 11.9% in March 2021 to 70.2% in July 2021 in Alberta. While this increase is primarily driven by vaccines, the study found that RBD antibody seropositivity rose from 9.4% in March to 20.2% in July among unvaccinated individuals, which would be attributable to infections. The findings are published in Infectious Diseases. This study was led by Drs. Carmen Charlton and Jamil Kanji based at Alberta Precision Laboratories.

These findings build upon an earlier paper (summarized here), which found an infection-acquired seropositivity rate of 4.6% (based on more than 900,000 samples) in the pre-vaccine era, between June 2020 and January 2021, in Alberta.

Key findings:

  • Among unvaccinated individuals:
    • RBD antibody seropositivity was highest in July 2021, among those aged 20-29, at 24.2%, followed by those aged 50-59 (23.2%).
    • Those with the lowest RBD antibody seropositivity rate at that time were aged 80 and older, at 12.0%.
    • In July 2021, the highest RBD antibody seropositivity rates were seen in the Grand Prairie and rural Alberta areas.
    • Unvaccinated individuals who were positive for RBD antibodies were more likely to live in lower-income neighbourhoods and were less likely to be married or in common-law relationships.
    • Unvaccinated RBD antibody positive individuals were also more likely to belong to a visible minority group.
    • 2% of unvaccinated individuals who were positive for spike antibodies had never had a diagnostic PCR test for SARS-CoV-2 done in the province. Of those who had a PCR test conducted, 24.2% were negative at the time of testing.
  • Among vaccinated individuals, RBD antibodies were present in 87.4-100% of individuals and were detectable up to 16 weeks following the first dose and up to 25 weeks following the second dose.

The present article is based on 60,632 blood samples from individuals of all ages collected between March 2021 and July 2021 (vaccine era) from six clinical laboratories across Alberta. Vaccination records were available for 98.8% of sampled tested.

*All seropositivity values have been adjusted for age and gender characteristics.