Introduction . Serological tests for COVID-19 are important in providing results for surveillance and supporting diagnosis . Investigating the serological response in COVID-19 patients with different disease severity is important for assessing the clinical utility of serological assays.Gap Statement . However, few studies have investigated the clinical utility of antibody assays for COVID-19 or differences in antibody response in association with disease severity.Aim . The study aimed to evaluate the clinical characteristics and clinical utility of VITROS SARS-CoV-2 antibody tests according to COVID-19 severity in patients in Japan.Methodology . We analysed 255 serum specimens from 130 COVID-19 patients and examined clinical records and laboratory data . Presence of total (IgA, IgM, and IgG) and specific IgG antibody for the spike 1 antigen of SARS-CoV-2 was determined using VITROS Anti-SARS-CoV-2 antibody tests.Results . Overall , 98 (75.4â %) and 32 (24.6â %) patients had mild and severe COVID-19, respectively . On admission , 76 (58.5â %) and 45 (34.6â %) patients were positive for total and IgG antibody assays . Among 91 patients at discharge , 90 (98.9â %) and 81 (89.0â %) were positive for total and IgG antibody, respectively . Clinical background and laboratory findings on admission, but not the prevalence or concentration of total or IgG antibody, were associated with disease prognosis . Total and IgG antibody intensities were significantly higher in severe cases than in mild cases in serum collected> 11 days after onset, but not within 10 days.Conclusion . VITROS Anti-SARS-CoV-2 total and IgG assays will be useful as supporting diagnostic and surveillance tools and for evaluation of humoral immune response to COVID-19 . Optimal prediction of disease prognosis is made from considering both clinical history and laboratory findings.