This is a summary, written by members of the CITF Secretariat, of:

Bazin R, Rochette S, Perreault J, Fournier M-J, Grégoire Y, Boivin A, Lewin A, Germain M, Renaud C. Development and use of a method based on the anti-N reactivity of longitudinal 1 samples to better estimate SARS-CoV-2 seroprevalence in a vaccinated population. medRxiv 2022.08.15.22278798; doi: https://doi.org/10.1101/2022.08.15.22278798.

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

In a preprint, not yet peer-reviewed, CITF-funded researchers developed a novel approach to detect SARS-CoV-2 anti-nucleocapsid antibodies. The ratio-based approach had a 95.2% sensitivity among both previously vaccinated and previously infected donors compared with 63.3% for the conventional approach.

Due to the lack of sensitivity of the conventional approach, which uses a seropositive threshold that distinguishes between anti-N seropositive from seronegative individuals, a new ratio approach was developed. It provided a more accurate estimate of individuals who had had a recent infection by obtaining longitudinal samples per individual to determine seropositivity during a given period.

Key findings:

  • A sensitivity of >95% was achieved with the ratio-based approach for detecting SARS-CoV-2 anti-nucleocapsid antibodies.
  • Using this approach, the adjusted seroprevalence was 9.7% from the beginning of the Omicron wave to January 18, 2022, 20.3% from January 18 to February 15, and 27.4% from February 15 to March 18, 2022. These percentages were much higher than values obtained using the conventional approach.

The ratio approach is more sensitive than the conventional approach at detecting seropositivity in vaccinated individuals with a recent history of SARS-CoV-2.

Plasma donations were collected by Héma-Québec from generally healthy individuals 18 years of age and older through the organization’s CITF-funded PLASCOV Biobank project. Donors in Québec had to have met the criteria of having received one or more vaccine doses against SARS-CoV-2, having had a confirmed SARS-CoV-2 infection between December 15, 2021 and March 20, 2022, and made two or more blood donations with specific timepoints.