Antibody response using six different serological assays in a completely PCR-tested community after a coronavirus disease 2019 outbreak-the CoNAN study

Abstract

Objectives: Due to a substantial proportion of asymptomatic and mild courses, many severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections remain unreported. Therefore, assessment of seroprevalence may detect the real burden of disease. We aimed to determine and characterize the rate of SARS-CoV-2 infections and the resulting seroprevalence in a defined population. The primary objective of the study was to assess SARS-CoV-2 antibody seroprevalence using six different IgG-detecting immunoassays. Secondary objectives of the study were: (a) to determine potential risk factors for symptomatic versus asymptomatic coronavirus disease 2019 courses, and (b) to investigate the rate of virus RNA-persistence.

Methods: CoNAN is a population-based cohort study performed in the community Neustadt am Rennsteig, Germany, which was quarantined from 22 March to 5 April after six SARS-CoV-2 cases were detected in the village's population. The SARS-CoV-2 outbreak comprised 51 cases and 3 deaths. The CoNAN study was performed from 13 May to 22 May 2020, 6 weeks after a SARS-CoV-2 outbreak.

Results: We enrolled a total of 626 participants (71% of the community population) for PCR and antibody testing in the study. All actual SARS-CoV-2 PCR tests were negative. Fifty-two out of 620 (8.4%) participants had antibodies against SARS-CoV-2 in at least two different assays. There were 38 participants with previously PCR-confirmed SARS-CoV-2 infection. Of those, only 19 (50%) displayed anti-SARS-CoV-2 antibodies. We also show that antibody-positive participants with symptoms compatible with a respiratory tract infection had significantly higher antibody levels then asymptomatic participants (EU-assay: median 2.9 versus 7.2 IgG-index, p 0.002; DS-assay: median 45.2 versus 143 AU/mL, p 0.002). Persisting viral replication was not detected.

Conclusions: Our data question the relevance and reliability of IgG antibody testing to detect past SARS-CoV-2 infections 6 weeks after an outbreak. We conclude that assessing immunity for SARS-CoV-2 infection should not rely on antibody tests alone.

Keywords: Antibody response; Immunity; Quarantine; Severe acute respiratory syndrome coronavirus 2.

Fig. 1

Flow chart of the CoNAN study. ∗ PCR from pharyngeal washes obtained during the CoNAN study in May 2020.

Fig. 2

A comparison of test performance between the six serological IgG assays in 600 participants for whom all six assays were performed. (a) Upset plot of all antibody-positive participants and (b) Upset plot of previously SARS-CoV-2 PCR-positive participants. Abbreviations: SN.2019-nCoV IgG kit (Snibe Co., Ltd., Shenzhen, China); EU..SARS-COV-2 IgG ELISA kit (Euroimmun, Lübeck, Germany); DS..SARS-CoV-2 S1/S2 IgG CLIA kit (DiaSorin, Saluggia, Italy); ED..EDI Novel Coronavirus SARS-CoV-2 IgG ELISA kit (Epitope Diagnostics Inc., San Diego, USA).

Fig. 3

Associations for reported clinical symptoms for the outcome ‘positive antibody status’ for (a) all participants; (b) previously SARS-CoV-2 PCR-positive participants and (c) previously SARS-CoV-2 PCR-negative participants. Odds ratio and corresponding 95% CI are derived from the logistic generalized estimation equations model adjusted for household clustering and sex and age (linear); the plots display the complete cases.

Fig. 4

Semi-quantitative test results of (a) all six assays from all participants with serology shown for previously PCR-positive (black dots) and PCR-negative (green circles) participants. Dashed grey-line indicates threshold for positive test results according to the manufacturer. (b–e) Semi-quantitative test results of participants with self-reported symptoms (with two-sided p values of the Wilcoxon–Mann–Whitney test): (b) any symptom, (c) cough, (d) taste/smell disorder and (e) fever. When results were below or above the detection limit of the assay, values were set to the respective lower or upper boundaries. Abbreviations: SN..2019-nCoV IgG kit (Snibe Co., Ltd., Shenzhen, China); EU..SARS-COV-2 IgG ELISA kit (Euroimmun, Lübeck, Germany); DS..SARS-CoV-2 S1/S2 IgG CLIA kit (DiaSorin, Saluggia, Italy); ED..EDI Novel Coronavirus SARS-CoV-2 IgG ELISA kit (Epitope Diagnostics Inc., San Diego, USA).