The humoral immune response more than one year after SARS-CoV-2 infection: low detection rate of anti-nucleocapsid antibodies via Euroimmun ELISA

Abstract

Purpose: Antibody assays against SARS-CoV-2 are used in sero-epidemiological studies to estimate the proportion of a population with past infection. IgG antibodies against the spike protein (S-IgG) allow no distinction between infection and vaccination. We evaluated the role of anti-nucleocapsid-IgG (N-IgG) to identify individuals with infection more than one year past infection.

Methods: S- and N-IgG were determined using the Euroimmun enzyme-linked immunosorbent assay (ELISA) in two groups: a randomly selected sample from the population of Stuttgart, Germany, and individuals with PCR-proven SARS-CoV-2 infection. Participants were five years or older. Demographics and comorbidities were registered from participants above 17 years.

Results: Between June 15, 2021 and July 14, 2021, 454 individuals from the random sample participated, as well as 217 individuals with past SARS-CoV-2 infection. Mean time from positive PCR test result to antibody testing was 458.7 days (standard deviation 14.6 days) in the past infection group. In unvaccinated individuals, the seroconversion rate for S-IgG was 25.5% in the random sample and 75% in the past infection group (P = < 0.001). In vaccinated individuals, the mean signal ratios for S-IgG were higher in individuals with prior infection (6.9 vs 11.2; P = < 0.001). N-IgG were only detectable in 17.1% of participants with past infection. Predictors for detectable N-IgG were older age, male sex, fever, wheezing and in-hospital treatment for COVID-19 and cardiovascular comorbidities.

Conclusion: N-IgG is not a reliable marker for SARS-CoV-2 infection after more than one year. In future, other diagnostic tests are needed to identify individuals with past natural infection.

Keywords: Antibody; COVID-19; Nucleocapsid; Seroprevalence; Spike.

Fig. 1

Distribution of anti-SARS-CoV-2-spike antibody ratios between different groups. a Violin plot showing the distribution of anti-spike-IgG signal ratios between individuals from a random sample (= control, N = 454) and individuals with PCR-proven prior infection (N = 217). b Shows the effect of vaccination status on the distribution of anti-spike-IgG signal ratios between groups. The groups are comprised of unvaccinated individuals in the control group (N = 145), unvaccinated individuals with prior SARS-CoV-2 infection (N = 48), vaccinated individuals in the control group (N = 298) and vaccinated individuals with prior infection (N = 163). c Information on the type of vaccination was available from 387 individuals from both groups. The anti-spike-IgG antibody response regarding vaccination with either viral vector vaccines, mRNA vaccines or after heterologous vaccination (viral vector vaccine followed by mRNA vaccine) is shown. The dotted lines represent the signal ratio cutoff of 1.1. A Mann–Whitney U test was used for statistical analysis in a and b. Kruskal–Wallis test was used in c. P values less than 0.001 are summarized with three asterisks, and P values less than 0.0001 are summarized with four asterisks. ns not significant