Persistence of SARS-CoV-2 N-Antibody Response in Healthcare Workers, London, UK

Prospective serosurveillance of severe acute respiratory syndrome coronavirus 2 in 1,069 healthcare workers in London, UK, demonstrated that nucleocapsid antibody titers were stable and sustained for <12 weeks in 312 seropositive participants. This finding was consistent across demographic and clinical variables and contrasts with reports of short-term antibody waning.

Prospective serosurveillance of severe acute respiratory syndrome coronavirus 2 in 1,069 healthcare workers in London, UK, demonstrated that nucleocapsid antibody titers were stable and sustained for <12 weeks in 312 seropositive participants. This fi nding was consistent across demographic and clinical variables and contrasts with reports of short-term antibody waning.
We observed no difference in seropositivity by sex, profession, performance of aerosol-generating procedures, employment in the emergency department, or immunocompromised status (Appendix  Table 2). Participants 25-34 years of age had higher odds of seropositivity than those 35-44 years of age (aOR 1.57, 95% CI 1.09-2.26), but little difference was seen among older age groups. Those working in intensive care units had lower odds of seropositivity than participants from other hospital departments (aOR 0.58, 95% CI 0.38-0.91).
log antibody titers remained stable over time in seropositive participants, and little within-individual variability was observed ( Figure). The general trend across all subgroups was a slight increase over time, although data are sparse for some groups.
We modeled trends beginning 4 weeks after the first positive antibody test. The mean weekly change was a 3.9% increase (95% CI 3.2%-4.6%). The model enables individual variability and thus estimates a distribution in trends, which ranged from a 0.5% decrease to an 8.5% increase per week, at 1 SD below/ above the mean.

Conclusions
In this study, N-antibody seropositivity was 29% among healthcare workers, and a small, sustained rise in antibody titers occurred over 12 weeks. The increase could be explained by the natural boosting of antibodies through repeated SARS-CoV-2 exposure; however, we saw no evidence of sporadic, sharp increases in antibodies in seropositive participants, and we observed little deviation from an overall linear trend. High initial seroprevalence and low subsequent seroconversion rates (Appendix Figures 1, 2) indicate that most exposures occurred before surveillance began. The low seroincidence after April might be attributable to changes in hospital infection control practices and national lockdown.
These findings demonstrate the short-term stability of N-antibody titers in healthcare staff, regardless of demographic or clinical differences. Seropositive participants not reporting any COVID-19 diagnosis or previous illness (even mild or atypical symptoms) demonstrated the same antibody trends as those who reported symptoms or laboratory-confirmed CO-VID-19, thereby supporting N-antibody testing as a reliable surveillance indicator. Although seroreversion was uncommon, such rates, if sustained, might be concerning in the long term.
Although cross-reactivity against the N protein has been observed and appears more prevalent than cross-reactivity against the spike (S) protein (E.M. Anderson, unpub. data, https://doi.org/10.1101/2020.1 1.06.20227215; C.F. Houlihan, unpub. data, https:// doi.org/10.1101/2020.06.08.20120584), the risk for false positives because of preexisting human coronavirus antibodies seems low on the basis of available data. The Elecsys assay demonstrated >99.5% specificity in 2 independent evaluations using large numbers of prepandemic control samples (3,4) and demonstrated high positive predictive value at an estimated 10% seroprevalence. Nonetheless, this study is limited by use of a single immunoassay, by selfreported data on COVID-19 diagnosis, and by limited testing early in the pandemic.
Several studies have demonstrated substantial declines in antibody titers over 3-5 months by using anti-S or anti-receptor-binding domain immunoassays (5)(6)(7)(8)(9). Although findings are not consistent across all reports (6,10), disparities could be explained by shorter follow-up periods that missed later decline. In contrast, the few studies conducting serial testing for >3 months by using N-antibody assays, particularly the Elecsys assay, report that titers remained steady (9) or increased (11; F. Muecksch, unpub. data, https://doi.org/10.1101/ 2020.08.05.20169128). These studies were limited by small sample sizes, single-site recruitment, and few time points with long sampling intervals. Our study replicates these findings in a large, multicenter cohort with frequent sampling and focuses on healthcare workers with mostly asymptomatic or mild disease, with robust statistical analysis to demonstrate consistent findings across all groups. These data can usefully inform serosurveillance strategies during the second wave.
For unknown reasons, N-antibodies appear highly stable in the short term, despite demonstrating no functional role; whether this stability would persist over longer follow-up periods remains to be answered. Although less useful as correlates of DISPATCHES immunity, N-antibodies could serve a critical role in serosurveillance as S-based vaccines are deployed, helping to distinguish infection-induced seroconversion from vaccine-induced seroconversion.