Serum samples from these people were collected. Methods for testing serum IgM and IgG antibodies to SARS-CoV-2 were previously described.50 COVID-19 IgM/IgG test kits contained recombinant SARS-CoV-2 antigens (spike protein and nucleocapsid protein) labelled with magnetic beads (tested on a fully automated chemiluminescence immunoassay analyzer) or colloidal gold (test card), anti-human IgM monoclonal antibody, and anti-human IgG monoclonal antibody. These test kits were reported to have high sensitivity and specificity27,50. According to the manufacturers, the sensitivity and specificity are ~90% and >99% for IgM, and ~98% and ~98% for IgG, respectively. Two physicians extracted the following data using data collection form from electronic medical records: demographic information such as age and sex, RT-PCR test date and results, COVID-19 IgM/IgG test date and results, date of symptom onset for COVID-19 patients, treatments received, and clinical outcomes. Another physician in the research team reviewed the collected data.