Time series analysis and mechanistic modelling of heterogeneity and sero-reversion in antibody responses to mild SARS‑CoV-2 infection

Abstract

Background: SARS-CoV-2 serology is used to identify prior infection at individual and at population level. Extended longitudinal studies with multi-timepoint sampling to evaluate dynamic changes in antibody levels are required to identify the time horizon in which these applications of serology are valid, and to explore the longevity of protective humoral immunity.

Methods: Healthcare workers were recruited to a prospective cohort study from the first SARS-CoV-2 epidemic peak in London, undergoing weekly symptom screen, viral PCR and blood sampling over 16-21 weeks. Serological analysis (n =12,990) was performed using semi-quantitative Euroimmun IgG to viral spike S1 domain and Roche total antibody to viral nucleocapsid protein (NP) assays. Comparisons were made to pseudovirus neutralizing antibody measurements.

Findings: A total of 157/729 (21.5%) participants developed positive SARS-CoV-2 serology by one or other assay, of whom 31.0% were asymptomatic and there were no deaths. Peak Euroimmun anti-S1 and Roche anti-NP measurements correlated (r = 0.57, p<0.0001) but only anti-S1 measurements correlated with near-contemporary pseudovirus neutralising antibody titres (measured at 16-18 weeks, r = 0.57, p<0.0001). By 21 weeks' follow-up, 31/143 (21.7%) anti-S1 and 6/150 (4.0%) anti-NP measurements reverted to negative. Mathematical modelling revealed faster clearance of anti-S1 compared to anti-NP (median half-life of 2.5 weeks versus 4.0 weeks), earlier transition to lower levels of antibody production (median of 8 versus 13 weeks), and greater reductions in relative antibody production rate after the transition (median of 35% versus 50%).

Interpretation: Mild SARS-CoV-2 infection is associated with heterogeneous serological responses in Euroimmun anti-S1 and Roche anti-NP assays. Anti-S1 responses showed faster rates of clearance, more rapid transition from high to low level production rate and greater reduction in production rate after this transition. In mild infection, anti-S1 serology alone may underestimate incident infections. The mechanisms that underpin faster clearance and lower rates of sustained anti-S1 production may impact on the longevity of humoral immunity.

Funding: Charitable donations via Barts Charity, Wellcome Trust, NIHR.

Keywords: SARS-CoV-2, Serology, Mathematical modelling, Sero-reversion.

Fig. 1

Longitudinal infection with SARS-COV-2 over 21 weeks across 731 healthcare workers. Results from testing for SARS-CoV-2 by cohort showing weekly PCR percentage positivity (weekly results, 95% CI) and seropositivity (cumulative percentage using combined anti-S1 IgG and anti-NP IgM/IgG, standard error).

Fig. 2

Longitudinal antibody responses across all timepoints in participants seropositive at any timepoint Individual participant data (left) and time ordered aggregate data (right) for Euroimmun anti-S1 antibody assay (N=145) (a–b) and Roche combined anti-NP antibody assay (N=150) (c–d) showing the heterogeneity in antibody responses between individuals and the differences in antibody kinetics between assays, with an earlier peak and decline in anti-S1 compared with anti-NP antibodies.

Fig. 3

Correlation of anti-S1 and anti-NP antibody measurements with neutralising antibody titres at 16–18 weeks. Comparison of neutralising antibody (nAb) titres represented as 50% inhibitory concentrations (IC) with Euroimmun anti-S1 levels (a) and Roche anti-NP levels (b) in 54 participants with nAb measurements and near-contemporaneous (±2 weeks) Euroimmun and Roche serology. R and p values by Spearman rank correlations.

Fig. 4

Mathematical modelling of kinetics of circulating anti-S1 and anti-NP antibodies. Model fit to aggregate (median) data from all seropositive participants for anti-S1 (N=92) (a) and anti-NP (N=86) (b) assays. Best fit model parameters for individual seropositive participants for half-life of antibody clearance (c), time to transition point of lower antibody production (d) and relative reduction in antibody production following this transition point (e). Horizontal lines (c–e) show median and interquartile range. (P values are provided for comparison of model parameters by Mann-Whiney tests).