The following is a summary of “Canadian antenatal COVID-19 seroprevalence study; population mapping of the COVID-19 pandemic utilizing stored antenatal sera,” published in the FEBRUARY 2023 issue of Obstetrics and Gynecology by Atkinson A, et al.

For a study, researchers sought to determine the distribution and rate of SARS-CoV-2 infection and vaccination-mediated immunity in Canada. The Canadian Antenatal Serological Survey used residual antenatal sera as a representative sample of reproductive-age females to assess SARS-CoV-2 seroprevalence across ten provinces at various pandemic stages.

The retrospective surveillance project had three phases. Phase One assessed seropositivity in three time periods over 2020 before vaccination implementation. Phase Two compared vaccination and natural immunity/infection, and Phase Three provided an up-to-date seropositivity assessment amidst emerging variants. In addition, postal code and age were recorded for comparison with PCR-positivity rates and regional distribution.

Before vaccination implementation, in Phase One (three time periods throughout 2020), seropositivity rates were 1.5–10 times higher than reported PCR-positive rates, and seropositivity was discovered in three provinces before the pandemic was declared. For example, raw anti-nucleocapsid antibody seroprevalence from 3,369 samples in British Columbia was 3.5% in Phase Two (November 15, 2021–December 3, 2021), and it was 88.1% for anti-spike antibodies. Anti-nucleocapsid antibodies peaked in Phase Three (100 samples/week, December 2021–early March 2022), reaching a maximum of 50% in the fourth week of February 2022, while anti-spike antibodies hit 90%.

The study concluded that antenatal seroprevalence helps assess SARS-CoV-2 spread and immunity nationally. The total seropositivity was below 6% in Phase One, indicating widespread vulnerability to SARS-CoV-2 before vaccine introduction in Canada, with underreporting by public health tracking of infections by approximately fourfold. The data from Phases Two and Three can help track natural infection and vaccination uptake as new variants emerge.