Immunoglobulin measurement
A total 180 saliva specimens and 181 peripheral blood specimens were obtained from COVID-19 patients with RT-PCR-confirmed prior SARS-CoV-2 infection, at different time points during hospitalization. Saliva specimens and peripheral blood specimens were also obtained from negative-control patients. The serum specimens were obtained from the supernatant of centrifuged peripheral blood at 3500 rpm for 5 min. The saliva specimens were also centrifuged and the supernatants were obtained to perform immunoglobulin detection. Before test, all specimens were inactivated at 56℃ for 30 min. Immunoglobulins against the SARS-CoV-2 surface spike protein receptor-binding domain (RBD) was measured by chemiluminescence kit (IgA, IgG, and IgM, Beijing Wantai Biotech, China) according to the manufacturer’s instruction in the Caris200 automatic chemiluminescence instrument. Fluorescence intensity was used to measure antibody concentration. The relative fluorescence of sample to control (COI) was used to estimate the result. The results ≥ 1 COI are positive, and the results < 1 COI are negative. The peak COI values of immunoglobulins were analyzed based on the results of relative fluorescence measurement.