Neutralizing antibodies (NAbs) tests v1

Antibody assays of IgM, IgG and surrogate isotype independent virusneutralizing antibody (sVNT) targeting receptor binding domain of Severe AcuteRespiratory Syndrome Coronavirus 2 (SARS-CoV-2) were employed in 97 real-timeReverse Transcription Polymerase Chain Reaction (RT-PCR) confirmed CoronavirusDisease 2019 (COVID-19) patients with varying severity admitted to KingChulalongkorn Memorial Hospital. Concordance rate was 100% regardless ofseverity, onset of symptoms and magnitude of viral load. Per available samples,antibodies appeared on the same day of symptom onset in one patient; one dayafter in 18 patients and two days after in 19 patients. In two patients,antibodies appeared as early as 4 days after infection (exposure). IgM and IgGwere evident in all patients’ first assay (within two days of admission). sVNTwas also evident within two days of admission in all but 3 patients. IgMusually remained positive during the entire course of hospital stay, where thelongest in this study was 32 days. Antibody assays were also applied to samplescollected at a State Quarantine premise from 77 asymptomatic Thais returningfrom Sudan in October. Virus was detected by real-time RT-PCR in 15 cases (day0=6, day 3=4, day 5=4 and day 9=1). Twenty-nine (including 11 RT-PCR positivecases) were antibody positive on day 0, while 4 PCR positive with antibodynegative on day 0 became antibody positive on day 14. Evaluation on antibodyresponse at days 7 or 10 is needed to help build a case to shorten length ofquarantine among negative cases.