Association of Anti-N and -S Seroprevalence in Asymptom atic, Mildly Symptomatic, and Symptomatic SARS-COV-2 Natural Infection

SARS-COV-2 infection is not always correlated with protection. Antibody seroprevalence in unvaccinated individuals, which is usually measured by N-specific antibodies, is not necessarily correlated with protection, while antibodies against S protein show a better correlation with protection due to its neutralizing epitopes. In this study, we tried to improve our conception of the hidden perspective of SARS-COV-2 in epidemiological reports and investigate anti-S antibody prevalence among anti-N antibody-positive asymptomatic and mildly symptomatic patients.

Blood samples were collected from asymptomatic or mildly symptomatic volunteer participants and symptomatic hospitalized patients with negative PCR results from May 30 to June 17, 2020. Detection of SARS-COV-2 antibodies was done using an ELISA kit targeting N or S protein.

Totally, 716 samples from volunteer participants and 81 samples from symptomatic hospitalized patients with negative PCR results were evaluated. The test performance-adjusted seroprevalence (95% CI) of SARS-COV-2 antibody was 17.3% (8.8-25.8%) for anti-N IgG in volunteers and 25.5% (12.8-39.7%) for anti-N and anti-S IgM in hospitalized patients. Among anti-N IgG positive infected individuals, 49.2% (21.4 and 78.8%) were anti-S antibody positive.

The results showed that SARS-COV-2 infection sometimes occurs in individuals without symptoms or with mild symptoms, but in more than half of them, the produced antibody is not protective. The findings of hospitalized patients showed that the combination of IgM assay with real-time PCR improved the disease diagnosis by more than 25% in cases with negative molecular test results.