Seropositivity rates among pregnant people were about four times higher than documented PCR-positive rates

A CITF-funded report published in March 2022 by the Canadian COVID-19 Population Serological Survey Utilizing Antenatal Serum Samples project leveraged existing blood samples collected through routine prenatal screening to assess immunity levels in the general population through the lens of the pregnant population. This Antenatal Serostudies report suggested that seropositivity rates among pregnant people were between 1.5- and 10-fold higher than concurrent PCR-positive rates, depending on the pandemic wave underway at the time. The study team calculated that PCR-based public health tracking systems underreported infections by four-fold, on average. The team also found that there were pregnant people in all provinces who had infection-acquired antibodies to SARS-CoV-2 in February 2020, showing that SARS-CoV-2 transmission was underway in Canada before the pandemic was declared. The compilation of this report was led by Dr. Deborah Money (University of British Columbia).

By March 2022, residual antenatal serum (blood from people who went in for prenatal tests) stored in 10 provincial laboratories over three time periods were tested: February 3-21, 2020, August 24-September 11, 2020, and November 16-December 4, 2020. In the second phase, a total of 8,601 collected samples were tested in four provinces between November 15, 2021-September 11, 2022. These data coincide with the emergence of the Omicron variant, which was first detected on November 25, 2021, in British Columbia.

Key findings:

• Seropositivity rates were between 1.5 and 10-fold higher than concurrent PCR-positive rates across the 10 provinces surveyed, depending on the pandemic wave at the time. This study team calculated that during the time periods sampled, PCR testing-based public health tracking systems in all provinces were underreporting infections by four-fold on average. Given that pregnant people are considered fairly representative of females in the age group of ~18-45 crossing urban/rural and diverse sociodemographic sectors, this is important public health surveillance information.
• Infection-acquired antibodies were found in pregnant women in the 10 provinces surveyed in February 2020, demonstrating viral transmission in Canada even before the pandemic was declared. Overall, seropositivity in the pre-vaccine era among pregnant women was under 6%, indicating widespread vulnerability of people to SARS-CoV-2 prior to the advent of vaccination in Canada.
• In later time periods (Omicron era), with extensive vaccine availability, of those who tested positive by PCR, 91% were fully vaccinated (at least two doses), indicating breakthrough infection. Overall, 25% of PCR-confirmed women were negative for infection-acquired antibodies (anti-N). Among them, 42% were infected six months or more prior to prenatal testing, showing antibody waning. The remainder may have been recently infected – within the three weeks prior to prenatal testing – and might not have had enough time to seroconvert. Among all PCR-confirmed individuals with serum samples collected within a suitable time frame from infection, 11% failed to seroconvert.