Development and use of a method based on the anti-N reactivity of longitudinal samples to better estimate SARS-CoV-2 seroprevalence in a vaccinated population

Abstract

Background: Serosurveys have been key to public health decision-making since the beginning of the SARS-CoV-2 pandemic. However, emerging evidence suggests that vaccination decreases the sensitivity of anti-nucleocapsid (N) serologies, making them less reliable to assess recently-acquired infections. We therefore developed and tested a new approach based on the ratio of the anti-N absorbance of longitudinal samples to overcome this limitation. Methods: Previously vaccinated repeat plasma donors infected between 12/15/2021 and 03/20/2022 were included if they made at least one donation pre-infection (reference sample) and one donation post-infection (test sample). All samples were tested using an in-house anti-N ELISA, the sensitivity of which had been determined prior to the vaccination era. Instead of using the previously determined signal cut-off (conventional approach), seropositivity was determined based on the ratio between the anti-N absorbance of the test and reference samples. The approach was tested in a real-world setting during three cross-sectional serosurveys carried out among plasma donors in Québec (Canada) from 01/17/2022 to 03/18/2022. Results: Using a cut-off ratio of 1.5, this approach had a sensitivity of 95.2% among the 248 previously vaccinated and infected donors compared with 63.3% for the conventional approach. When tested in a real-world setting, the ratio-based approach yielded an adjusted seroprevalence of 27.4% (95% confidence interval [CI]=23.8%-30.9%) at the latest time point considered, compared to 15.1% (95% CI=12.2%-18.0%) for the conventional approach. Conclusions: This article describes a new and highly-sensitive approach that captures a significantly greater proportion of vaccinated individuals with a recent history of SARS-CoV-2 infection.

Humoral Responses against BQ.1.1 Elicited after Breakthrough Infection and SARS-CoV-2 mRNA Vaccination

“…The study was conducted in 63 individuals (25 males and 38 females; age range: 24–84 years). In total, 20 of these individuals had recent breakthrough infection with an Omicron sublineage (9 males and 11 females; age range: 24–67 years), i.e., as determined by the increase in anti-N levels between W4-Va3 and M4-Va3 or between M4-Va3 and W4-Va4 (ratio M4-Va3/W4-Va3 and/or W4-Va4/M4-Va3 higher than 1.5) using a recently described analytical approach [ 24 ] ( Figure 2 ). For the other donors (16 males and 27 females; age range: 31–84 years), we did not observe a significant increase in the anti-N levels, although some of them have a history of infection (to our knowledge, 12 donors had a history of infection: 3 were infected during the first wave of COVID-19 in winter/spring 2020, and 9 of them were tested anti-N positive at W4-Va3).…”

“…Because breakthrough infection (BTI) strongly improved humoral responses, and may impact the responses measured after vaccination, we also measured the anti-nucleocapsid (N) level at these three time points to determine if the donors have been infected between their third and fourth doses of vaccine (by an Omicron sublineage). Based on anti-N results, the donors were separated in two groups, donors without recent BTI for whom we did not observe an increase in anti-N over time, and donors with recent BTI for whom we measured a significant increase in anti-N between the third and fourth doses of vaccine ( Figure 2 A), according to a recent study [ 24 ]. Basic demographic characteristics of the cohort are summarized in Table 1 .…”

Humoral responses against BQ.1.1 elicited after breakthrough infection and SARS-CoV-2 mRNA vaccination

“…20 of these individuals had recent breakthrough infection with an Omicron sublineage (9 males and 11 females; age range: 24-67 years), i.e. as determined by the increase in anti-N levels between W4-Va3 and M4-Va3 or between M4-Va3 and W4-Va4 using a recently described analytical approach [18] (Figure S1). For the other donors (16 males and 27 females; age range: 31-84 years), we did not observe a significant increase of the anti-N levels, although some of them have a history of infection.…”