SARS-CoV-2 infection serology: a useful tool to overcome lockdown?

Abstract

The outbreak of 2019 novel coronavirus disease (Covid-19) caused by SARS-CoV-2 has spread rapidly, inducing a progressive growth in infected patients number. Social isolation (lockdown) has been assessed to prevent and control virus diffusion, leading to a worldwide financial and political crisis. Currently, SARS-CoV-2 RNA detection in nasopharyngeal swab takes place by real-time PCR (RT-qPCR). However, molecular tests can give some false-negative results. In this context, serological assays can be useful to detect IgG/IgM antibodies, to assess the degree of immunization, to trace the contacts, and to support the decision to re-admit people at work. A lot of serological diagnostic kits have been proposed on the market but validation studies have not been published for many of them. The aim of our work was to compare and to evaluate different assays analytical performances (two different immunochromatographic cards, an immunofluorescence chromatographic card, and a chemiluminescence-automated immunoassay) on 43 positive samples with RT-qPCR-confirmed SARS-CoV-2 infection and 40 negative control subjects. Our data display excellent IgG/IgM specificities for all the immunocromatographic card tests (100% IgG and 100% IgM) and for the chemiluminescence-automated assay (100% IgG and 94% IgM); IgG/IgM sensitivities are moderately lower for all methods, probably due to the assay viral antigen's nature and/or to the detection time of nasopharyngeal swab RT-qPCR, with respect to symptoms onset. Given that sensitivities (around 94% and 84% for IgG and IgM, respectively) implicate false-negative cases and given the lack of effective vaccines or treatments, the only currently available procedure to reduce SARS-CoV-2 transmission is to identify and isolate persons who are contagious. For this reason, we would like to submit a flowchart in which serological tests, integrated with nasopharyngeal swab RT-qPCR, are included to help social and work activities implementation after the pandemic acute phase and to overcome lockdown.

Keywords: Immunochemistry; Viral infection.

Fig. 1. Schematic of SARS-CoV-2 Immunochromatographic and immunofluorescence Card Test workflow and assay readout.

a Three detection lines are on the test cassette: a control (c) line appears when serum sample flows through the card; SARS-CoV-2 antibodies presence will be indicated by a colored line in the IgG and/or IgM test line regions. If C line does not appear, the test is invalid and should be repeated. b Samples, mixed with a buffer solution containing fluorescently labeled SARS-CoV-2 recombinant proteins, form antigen–antibody conjugates. The conjugates are added to the sample well in the test card and captured by nitrocellulose-coated goat-anti-human IgM or IgG antibodies. The resulting immunocomplexes are detected by a fluorescence detector. Positive cut-off values are as follows: anti-SARS-CoV-2 IgG positive>1,2T/C; anti-SARS-CoV-2 IgM positive > 1,3T/C.

Fig. 2. Anti-SARS-CoV-2 serological tests ROC curves.

Card 3 IgG and IgM results are shown in a (AUC 0.981 and 0.921, respectively); CLIA IgG and IgM results are shown in b (AUC 0.997 and 0.943, respectively).

Fig. 3. Flowchart proposal to overcome lockdown.

Path 1 describes serological IgG/IgM assays on asymptomatic workers: in case of negative results, after a 2-week preventive quarantine, a second negative IgG/IgM serological test is required to return to work; in case of positive results, two consecutive nasopharyngeal swabs RT-qPCR are mandatory. Path 2 describes serological IgG assays on Covid-19 convalescent workers or IgG/IgM-positive asymptomatic workers, after two consecutive negative nasopharyngeal swabs RT-qPCR, to detect natural immunization span.