Evaluation of anti-nucleocapsid level variation to assess SARS-CoV-2 seroprevalence in a vaccinated population

Serosurveys have been key to public health decision-making since the beginning of the SARS-CoV-2 pandemic. However, several studies have uncovered that vaccination blunts the anti-nucleocapsid (N) response to a subsequent infection, which hinders the ability of serologic assays (including commercial ones) to detect recent infections. We therefore developed a new analytical approach to increase the sensitivity of detection of infection in vaccinated individuals.

Two samples were obtained from 248 SARS-CoV-2-positive (PCR-confirmed), vaccinated donors: one before the infection (reference sample) and one after (test sample). All samples were tested using an in-house, anti-N enzyme-linked immunosorbent assay (ELISA) which had a sensitivity of 98.1% before the mass vaccination campaign. Instead of applying a seropositivity threshold based on a single absorbance value (i.e. conventional approach), seropositivity was determined based on the ratio between the anti-N absorbance of the test and reference samples.

The sensitivity of the new approach to detect infection in vaccinated individuals was 95.2% using a cut-off of 1.5 for the anti-N ratio, whereas that of the conventional approach was 63.3%.

The new analytical approach described herein captured a significantly greater proportion of vaccinated individuals with a known history of SARS-CoV-2 infection than the conventional approach used in most serosurveys.