Antibody trends in the first 11 months of 2021: Canadian Blood Services

In a paper published in Microbiology Spectrum, researchers with Canadian Blood Services (CBS) point to the changing patterns of infection-acquired and vaccine-induced seroprevalence, observed between January and November 2021. Seropositivity from infection remained low overall, albeit disproportionally higher among racialized donors and those aged 17-24. Antibodies targeting the spike protein (against which vaccines were designed) increased dramatically in July, following widespread vaccine roll-out, and started to wane come September. The study is led by Drs. Sheila O’Brien, Chantale Pambrun, and Steven Drews. CBS has routinely tested samples from blood donors to provide data about COVID-19 immunity, which have been used to inform public health policy at the national and provincial levels.

Key results:

• Infection-acquired seropositivity (as evidenced by anti-nucleocapsid antibodies) increased gradually, from 2.2% in January 2021 to 5.0% in November 2021 (before the arrival of Omicron).
• Concurrently, antibodies targeting the spike protein (which could reflect vaccine-induced or infection-acquired immunity) skyrocketed from 2.8% in January (when vaccines were not yet widely available) to 98.5% in November (when most Canadians were vaccinated with at least one dose).
• Socio-demographic groups that were more likely to be seropositive from previous infection were racialized groups, younger adults (17-24), and those living in lower income neighbourhoods (as measured by the Social Deprivation and Material Deprivation Indices, based on postal codes).
• Socio-demographic groups that were more likely to be positive for spike antibodies included older adults (age 70 and older), females, and those living in more affluent neighbourhoods.
• The median concentration of spike antibodies, measured in arbitrary units per millilitre (U/mL), remained virtually unchanged from January 2021 to May 2021 (less than 100 U/mL). Concentrations ticked up in June 2021 (126 U/mL) and then rose dramatically in July 2021 (over 2,500 U/mL). In August 2021, concentrations remained very high; however, the following month, they began to decline, initially among older age groups who were among the first to be fully vaccinated (age 70+). By November 2021, spike concentrations were falling across all age brackets, suggesting antibody wane. This coincided with the priority-based introduction of third (booster) doses.

In 2021, prior to the arrival of Omicron, seropositivity from infection remained low (<5%) in Canada, while most adults acquired spike-antibody immunity to the virus, primarily through vaccination.

This analysis captures data from blood donors across all provinces, except Quebec, from January 1 to November 24, 2021 – immediately prior to the arrival of Omicron in Canada. A total of 149,522 blood donations were profiled for antibodies targeting SARS-CoV-2 nucleocapsid (proxy for past infection) and spike (proxy for vaccination but also past infection). This time period encompasses the third pandemic wave in March – April 2021 and the Delta-fuelled fourth wave, which began in August and was subsiding by November 2021.

It should be noted that individuals who choose to donate blood are generally in good health and are more likely to live in populous urban areas. Percentages were adjusted for test characteristics and population distribution.

Explore our interactive webpage updated every month, featuring the latest aggregated data gathered by the Canadian Blood Services and Héma-Québec on SARS-CoV-2 seroprevalence in Canada.