A recent serosurvey conducted by Héma-Québec estimates that 27.4% of adults in the province developed infection-acquired antibodies to SARS-CoV-2 between January and mid-March 2022, during the first Omicron wave. Based on testing of plasma from donors enrolled in the CITF-funded PLASCOV project, infection-acquired seropositivity rose steadily throughout the sampling period: from 9.7% in mid-January to 27.4% by mid-March. Donors aged 18 to 24 had the highest levels of infection. These latest blood donor data reveal the full extent of SARS-CoV-2 infections, which are more than double that reported by PCR-confirmed case counts.

The latest report includes 568 people who donated plasma between January 17 to 18, including 237 of whom donated again from February 14 to 15, along with an additional 259 donors, for a total of 496 for that timepoint. It also includes 554 unique individuals who donated from March 16 to 18, 2022.

Key findings:

  • Recent infection-acquired seropositivity (as evidenced by a rise in the level of antibodies against the nucleocapsid protein in paired samples obtained from study participants, one obtained just before the Omicron wave and the second during the wave) increased steadily throughout the sampling period from 9.7% in mid-January, more than doubling to 20.3% by mid-February, and ultimately rising to 27.4% by mid-March.
  • The infection-acquired seropositivity rate of 27.4% by mid-March is more than double that of cumulative PCR-confirmed case counts in the province, based on an analysis conducted by Héma-Québec.
  • When extrapolated to the entire province, an infection-acquired seropositivity rate of 27.4% corresponds to around 1.9 million adults in Quebec having been infected with SARS-CoV-2 during the initial Omicron wave. This number jumps to about 2.4 million people infected with SARS-CoV-2 in Quebec if the results in adults are extrapolated to include the pediatric population.
  • The Montreal region experienced infection-acquired seropositivity rates of 13.8% in mid-January, 22.0% in mid-February and 36.4% in mid-March. While the first two months were in line with the rest of the province, the latter month is considerably higher than the rest of Quebec for that time period (24.2%). It should be noted, however, that donors from Montreal and its surrounding areas were under-represented in the study.
  • The proportion of donors with infection-acquired antibodies decreased according to age, from younger to older adults: 40.3% of donors 18 to 24 years old, 38.1% of donors aged 25-39 years old, 27.9% of donors aged 40-59 years old, and 18.7% of donors aged 60 and older (data for mid-March).
  • Donors aged 60 and older experienced the largest increase in infection-acquired seropositivity over time across all age categories; a 6.6 time increase from 2.8% in January to 18.7% by mid-March.
  • Rates of infection-acquired seropositivity did not differ significantly between socioeconomic quintiles (33.3% for residents of lower-income neighbourhoods vs. 28.2% for residents of higher-income neighbourhoods).

It should be noted that donors with a known active SARS-CoV-2 infection are not eligible to donate blood until their infection and symptoms have cleared (at least 14 days).

Seropositivity percentages were adjusted for test characteristics and population distribution.

Read the press release here.

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