Héma-Québec estimates that 62% of adults in Quebec had been infected with the Omicron variant by the end of August 2022

Héma-Québec estimates that 62% of Quebecers who donated plasma had acquired antibodies following an infection with Omicron by the end of August 2022. Plasma donors were evaluated over three periods of the Omicron wave: December 2021 to March 2022, March to June 2022, and June to August 2022. For each donor, Héma-Québec assessed whether the level of anti-N antibodies increased during a given period, which indicates a recent infection using the “ratio-based” approach. This new approach leverages longitudinal samples obtained from frequent plasma donors and was more sensitive than the “conventional approach” that uses a single measurement.

Key findings:

• The ratio-based approach was used to determine anti-N seropositivity based on the ratio between the absorbance of the test and reference samples.
• Among 248 donors with a PCR-confirmed Omicron infection, 95.2% had a significant increase in anti-N levels using the ratio-based approach, whereas only 63.3% had an elevated anti-N measurement using the conventional approach.
• Among 558 donors evaluated in August, 53.2% had a significant increase in anti-N antibodies using the ratio-based approach, whereas only 35.3% had anti-N antibodies according to the conventional approach.
• Accounting for the rates of seroconversion from December 2021 to March 2022 (23.3%), from March to June 2022 (20.9%), and from June to August 2022 (19.9%), the cumulative rate of infection for the Omicron wave reaches 62.1%. This higher cumulative rate (compared to the December to August evaluation) is explained by the seroreversion of the anti-N signal in some donors over this longer period.
• Montreal and its surrounding area had a weighted seroconversion rate of:
  • 20.1% between December 2021 and March 2022
  • 17.1% between March and June 2022
  • 11.8% between June and August 2022
• The other regions of Quebec showed a similar trend, but with higher rates.
• Men made up a greater portion of the sample than women, and donors from the Greater Montreal Area were underrepresented. Nearly all the plasma donors were Caucasian.
• It should be noted that donors with a known active SARS-CoV-2 infection cannot donate blood until their infection and symptoms have resolved. The percentages of seropositivity were adjusted for test characteristics and population distribution.

From December 2021 to August 2022, the Omicron variant seems to have infected close to two-thirds of frequent plasma donors. This figure likely reflects the extent of the Omicron wave in the general adult population of Quebec.

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.