Strategic Anti-SARS-CoV-2 Serology Testing in a Low Prevalence Setting: The COVID-19 Contact (CoCo) Study in Healthcare Professionals

Abstract

Background: Serology testing is explored for epidemiological research and to inform individuals after suspected infection. During the coronavirus disease 2019 (COVID-19) pandemic, frontline healthcare professionals (HCP) may be at particular risk for infection. No longitudinal data on functional seroconversion in HCP in regions with low COVID-19 prevalence and low pre-test probability exist.

Methods: In a large German university hospital, we performed weekly questionnaire assessments and anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) immunoglobulin G (IgG) measurements with various commercial tests, a novel surrogate virus neutralisation test, and a neutralisation assay using live SARS-CoV-2.

Results: From baseline to week 6, 1080 screening measurements for anti-SARS CoV-2 (S1) IgG from 217 frontline HCP (65% female) were performed. Overall, 75.6% of HCP reported at least one symptom of respiratory infection. Self-perceived infection probability declined over time (from mean 20.1% at baseline to 12.4% in week 6, p < 0.001). In sera of convalescent patients with PCR-confirmed COVID-19, we measured high anti-SARS-CoV-2 IgG levels, obtained highly concordant results from enzyme-linked immunosorbent assays (ELISA) using e.g. the spike 1 (S1) protein domain and the nucleocapsid protein (NCP) as targets, and confirmed antiviral neutralisation. However, in HCP the cumulative incidence for anti-SARS-CoV-2 (S1) IgG was 1.86% for positive and 0.93% for equivocal positive results over the study period of 6 weeks. Except for one HCP, none of the eight initial positive results were confirmed by alternative serology tests or showed in vitro neutralisation against live SARS-CoV-2. The only true seroconversion occurred without symptoms and mounted strong functional humoral immunity. Thus, the confirmed cumulative incidence for neutralizing anti-SARS-CoV-2 IgG was 0.47%.

Conclusion: When assessing anti-SARS-CoV-2 immune status in individuals with low pre-test probability, we suggest confirming positive results from single measurements by alternative serology tests or functional assays. Our data highlight the need for a methodical serology screening approach in regions with low SARS-CoV-2 infection rates.

Trial registration: The study is registered at DRKS00021152.

Keywords: COVID-19; Coronavirus; Healthcare professionals; Humoral immunity; Infection; Pandemic; SARS-CoV-2; Serological testing; Virus.

Fig. 1

Design of the CoCo study. The CoCo 1.0 cohort comprises 217 frontline HCP from emergency departments, infectious and pulmonary disease inpatient units, ICUs, pediatric departments and other units involved in COVID-19 patient care for weekly serologic screening for SARS-CoV-2 during the first 2 months followed by monthly testing. CoCo 2.0 cohort enrolment started in May 2020 to recruit at least an additional 1000 HCP from other clinical departments of Hannover Medical School for serologic assessments every 6 months

Fig. 2

Self-perceived infection risk over time. Reduction of mean self-perceived infection risk of all CoCo 1.0 cohort participants answering this question over the first 6 weeks. Bars display mean + SEM, *p < 0.05, ***p < 0.001

Fig. 3

Consistency of seropositivity rates of the different serological testing systems applied in CoCo 1.0 cohort. Results of the anti-SARS-CoV-2 S1 IgG versus IgA ELISA (a), anti-SARS-CoV-2 S1 IgG versus anti-SARS-CoV-2 NCP IgG ELISA (b), anti-SARS-CoV-2 S1 IgG ELISA versus the WANTAI anti-SARS-CoV-2 antibody rapid test (c), and anti-SARS-CoV-2 NCP IgG ELISA versus WANTAI anti-SARS-CoV-2 antibody rapid test (d). Red dots represent positive results (IgG ratio > 1.1, positive band, respectively), yellow dots represent borderline positive results (IgG ratio 0.8–1.1), and green dots represent negative results (IgG ratio < 0.8, no band, respectively)

Fig. 4

Serology results of eight HCP (1–8) in the CoCo 1.0 cohort with at least one positive or borderline positive anti-SARS-CoV-2 S1 IgG ELISA during the observation period. All samples from HCP with at least one positive or borderline result at any time point (HCP 1–8) were measured on one ELISA plate. Anti-SARS-CoV-2 S1 IgA is depicted in red, anti-SARS-CoV-2 S1 IgG depicted in blue. Results of anti-SARS-CoV-2 NCP (NCP), neutralisation assay (NA), or SARS-CoV-2 antibody rapid test (RT) from selected samples are indicated as positive or negative. The results of the neutralisation assay at IC₅₀ are given as 1:2 (+), 1:8 +, 1:32 and 1:64 ++, 1:512 +++

Fig. 5

Inhibition in the sVNT compared to neutralisation activity in the plaque assay. a Sera of 13 convalescent patients with PCR-confirmed COVID-19 with various neutralisation activity in the plaque assay (IC₅₀, 1:16 to ≥ 1:1024, red lines as indicated in the legend) are depicted in according to their percent age inhibition activity in the sVNT at various dilutions as indicated. b Increase of inhibition in the sVNT during seroconversion [week (W) 1–5] of HCP1 and rise in neutralisation activity in the plaques assay as depicted by the lines. c Inhibition results obtained in the sVNT with sera from HCP2–8, which had least one positive or equivocal positive anti-SARS-CoV-2 S1 IgG ELISA result. None of these sera revealed significant neutralisation activity in the plaque assay (IC₅₀ ≤ 1:2)