This is a summary, written by members of the CITF Secretariat, of:Bazin R, Rochette S, Perreault J, Fournier MJ, Grégoire Y, Boivin A, Lewin A, Germain M, Renaud C. Evaluation of anti-nucleocapsid level variation to assess SARS-CoV-2 seroprevalence in a vaccinated population. Infect Dis (Lond). 2023 Apr 11:1-6. doi:

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

In a study now published in Infectious Diseases, CITF-funded researchers from Héma-Québec reported on the development of a novel analytical approach to detect recent SARS-CoV-2 infections based on 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 used in most serosurveys. This study was led by Drs. Marc Germain and Renée Bazin.

Vaccination blunts the anti-nucleocapsid response to a subsequent infection, which hinders the ability of serologic assays to detect recent infections. The conventional approach uses a seropositive threshold that distinguishes between anti-N seropositive from seronegative individuals. The researchers developed a new ratio approach to overcome its lack of sensitivity. It provides a more accurate estimate of individuals with a recent infection by examining longitudinal samples for each individual to determine seropositivity during a given period.

Key findings:

  • Using the conventional analytical approach, none of the reference (pre-infection) samples and 157 (63.3%) test samples (from individuals with a PCR-confirmed recent infection) were seropositive. This indicated that about one third of infections would be missed using the conventional analytical approach.
  • A sensitivity of >95% was achieved for the detection of recent infections with the ratio-based 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 having made two or more blood donations with specific timepoints.