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.