key: cord-1032982-atmuj1ki
authors: Wellinghausen, Nele; Plonné, Dietmar; Voss, Meike; Ivanova, Ralitsa; Frodl, Reinhard; Deininger, Susanne
title: SARS-CoV-2-IgG response is different in COVID-19 outpatients and asymptomatic contact persons
date: 2020-07-06
journal: J Clin Virol
DOI: 10.1016/j.jcv.2020.104542
sha: 49561654cf2f862fdbad15d6a031235b3bcc6dd9
doc_id: 1032982
cord_uid: atmuj1ki

Commercially available immunoassays have been developed for sensitive and specific detection of antibodies against SARS-CoV-2. While a fast and reliable IgG response has been reported for samples from hospitalized COVID-19 patients, less is known about ambulatory patients. We evaluated the SARS-CoV-2-IgG response by the Anti-SARS-CoV-2-ELISA IgG (Euroimmun) in a defined cohort of SARS-CoV-2-PCR-confirmed outpatients and asymptomatic contact persons including 137 serum samples from PCR-confirmed outpatients (n = 111) and asymptomatic but PCR-positive contact persons (n = 26) sent to our laboratory as part of routine diagnostics for determination of SARS-CoV-2-IgG. Overall positivity rate for SARS-CoV-2-IgG was 81.1% in outpatients (irrespective of sampling before or after day 21 after onset of symptoms) but significantly lower in asymptomatic contact persons (15.4%, p < 0.0001). In contact persons without symptoms the ct values of the PCR assays were significantly higher (5 to 7 threshold cycles) than in outpatients, and ct values were significantly negative correlated to the SARS-CoV-2-IgG ratio, suggesting a lower viral load as a possible explanation for lower rate of seropositivity. In summary, our study shows that serological response to SARS-CoV-2 in outpatients including asymptomatic persons is less pronounced than in hospitalized patients. Further controlled studies are urgently needed to determine serological response in outpatients and asymptomatic persons since this is the main target population for seroepidemiological investigations.

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Within the third week after onset of symptoms SARS-CoV-2-IgG were detected in up to 100% of hospitalized patients by use of various commercial immunoassays (2;5-10) . It has been shown that SARS-CoV-2-IgG titers were higher in critically ill compared to less critically ill patients and that severely ill patients seroconverted earlier than those with mild disease (5;11;12) . Therefore, it might be assumed that serological response in outpatients with a less severe clinical course of COVID-19 differs from that of hospitalized patients. Outpatients and milder infected or even asymptomatic contact persons are, however, the main target population for screening for SARS-CoV-2 antibodies in order to evaluate disease epidemiology. Moreover, this group represents the vast majority of patients requesting SARS-CoV-2-IgG testing in our laboratory. Therefore, we evaluated the SARS-CoV-2-IgG response in outpatients and asymptomatic contact persons with past SARS-CoV-2 infection confirmed by RT-PCR.

Serum samples were sent to our laboratory from ambulatory patients for determination of SARS-CoV-2-IgG. The MVZ Labor Ravensburg is private laboratory serving a large number of private practices and hospitals in Southwest Germany as well as most coronavirus test center in the region. All serum samples sent to our laboratory for SARS-CoV-2-IgG determination between March 24 th and May 6 th 2020 from outpatients with a positive result of SARS-CoV-2-RT-PCR in a nasopharyngeal swab (at least 7 days before serum collection) were considered for analysis (n = 158). Information about clinical J o u r n a l P r e -p r o o f symptoms, day of onset of symptoms, and past hospital treatment for COVID-19 was obtained.

Patients with past hospital treatment for COVID-19 (n = 11) and patients in whom clinical information could not be obtained (n = 10) have been excluded from analysis. Out of the remaining 137 patients, 111 patients had clinically and PCR-confirmed, ambulatory treated SARS-COV-2 infection and fulfilled the clinical diagnostic criteria of the Robert-Koch-Institut (www.rki.de). All had recovered at the time point of blood collection. 26 persons had no clinical symptoms but were PCR-positive due to contact with PCR-confirmed COVID-19 patients. (Table 1) . Seropositivity rates of outpatients obtained up to day 20 and after day 20 after onset of symptoms were similar (81.8% versus 81.0%, Table 1 ).

Serum samples of 26 asymptomatic PCR-confirmed contact persons were sampled between day 9 and 56 (median day 29) after the day of the PCR-positive swab. The positivity rate of SARS-CoV-2-IgG amounted to 15.4% (4/26), which was significantly lower than that of the outpatients (p < 0.001). The positivity rate tended to be higher at a later time point of serum sampling (18.8% after day 20 versus 10 .0% before day 20 of PCR-positive swab, overall 15,4%, Table 1 ), but the difference was not significant (p = 0.547). The day of the first PCR-positive swab was also recorded in all outpatients and the median was identical to that of the asymptomatic contact persons (median day 29). Previous publications have demonstrated that the humoral immune response towards SARS-CoV-2 is dependent on the duration and magnitude of viral antigen exposure (15;16) . Therefore, it may be postulated that the group of asymptomatic contact persons have been exposed to a lower amount of viral antigen. For this reason, we analysed the ct values of the RT-PCR runs and, indeed, found significantly higher ct values, i.e. significantly lower viral loads, in the swabs obtained from asymptomatic contact persons compared to outpatients. In addition, ct values and the SARS-CoV-2-IgG ratio were significantly negative correlated in both groups. Due to the correlation between the ct value and SARS-CoV-2 IgG, the antibody status could be determined with a certain sensitivity and specificity using a ct threshold (Ct = 34) value determined by ROC analysis. Below the threshold value the IgG result is positive (sensitivity 94%) and above the threshold value it is negative (specificity 72%).

(c) The possibility of false positive RT-PCR results has to be taken into account. Contamination of samples can never completely be excluded, but the following reasons make this explanation unlikely:

First, samples that were investigated on the cobas® 6800 analyzer were mainly directly put into the analyzer without prior opening in the laboratory, second, we retested a large collection of swabs with J o u r n a l P r e -p r o o f a weak positive result in an E-gene-specific PCR with another different PCR assay and revealed consistent results (data not shown), and, third, the majority of swab samples was positive for two SARS-CoV-2 gene targets. Nevertheless, our study has some limitations: The cohort included in our study represented a well defined but not prospectively acquired collection of serum samples, and single patients may have been missed that putatively had PCR-tests in another laboratory. The time point of swab sampling in asymptomatic persons and outpatients were in median comparable but individual differences in swab sampling, use of different swab and transport systems cannot be excluded.

In summary, our study shows that SARS-CoV-2-IgG antibodies are found significantly less frequently in PCR-positive asymptomatic contact persons compared to PCR-positive outpatients. Significantly higher ct values in contact persons and the negative correlation of the ct values with the SARS-CoV-2-IgG ratio suggests a lower viral load as a possible explanation for lower rate of seropositivity in asymptomatic contact persons. Further studies are needed to determine serological response in mildly infected and asymptomatic persons since this is the main target population for seroepidemiological studies.

NW designed the study and wrote the manuscript. RF analyzed the RT-PCR assays. SD, MV, RI, and NW obtained clinical information and analyzed data. DP did statistical analysis. All authors were involved in scientific discussion.

There are no conflicts of interest by all authors.

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Performance of six SARS-CoV-2 immunoassays in comparison with microneutralisation

Antibody profiling of COVID-19 patients in an urban low-incidence region in Northern Germany

Viral dynamics in mild and severe cases of COVID-19

Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a retrospective cohort study