Moreover, the clinical usefulness of antibody screening also needed to be demonstrated. (required mechanical ventilatory support)), and serially measured the titers of IgM, IgG, and IgA against the nucleocapsid protein, spike protein, Calcineurin Autoinhibitory Peptide and receptor-binding website of SARS-CoV-2 until day time 12 after sign onset. The titers of all the measured antibody reactions were higher in severity group 2b and 3, especially severity group 2b, as early as at one week after sign onset. Addition of data from antibody screening improved the ability of analysis models constructed using a machine learning technique to distinguish severity group 2b and 3 from severity group 1 and 2a. These models constructed with non-vaccinated COVID-19 individuals could not become applied to the instances of breakthrough infections. These results suggest that antibody screening might help physicians determine non-vaccinated COVID-19 individuals who are likely to require admission to an intensive care unit. Keywords: COVID-19, severity, machine learning, IgM, IgG, IgA, nucleocapsid protein, spike protein Intro Coronavirus disease 2019 (COVID-19), Calcineurin Autoinhibitory Peptide caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), exhibits a wide medical spectrum, ranging from an asymptomatic state to severe disease requiring mechanical respiratory support. For proper triage of the individuals and appropriate use of medical resources for individuals with COVID-19, it is important to identify suitable biomarkers/diagnostic systems for predicting the maximum severity of COVID-19 FMN2 in the early phase of the disease. Until date, several demographic characteristics and medical features, including laboratory data, have been reported to be associated with the severity of COVID-19, including male sex, advanced age, underlying hypertension, diabetes, and cardiovascular disease, positive smoking history (1, 2), and serum CRP and D-Dimer levels (3, 4). In addition to the aforementioned guidelines, the titers of antibodies against SARS-CoV-2 have also been reported by several studies to be associated with the disease severity in COVID-19 individuals. While, in general, studies possess reported positive associations between SARS-CoV-2 antibody titers and the medical disease severity and/or Calcineurin Autoinhibitory Peptide laboratory data such as the serum CRP, a few studies have refused the living of a positive association between the antibody titers and the severity of COVID-19 (5C7). The timing of measurement of the antibodies assorted among these studies; while the measurements were made in a rather acute phase of the disease, that is, within 2 weeks from the onset of symptoms, in some studies (8C20), in others, they were made in the later on phases of the disease (8C10, 13, 16C19, 21C32), at the time of admission to the hospital (33, 34), or at arbitrary occasions (35C37). Previous studies have shown positive associations between the medical severity of COVID-19 and variously measured antibody responses, including the neutralizing antibody titers (8, 10, 17, 20, 21, 26, 28, 31, 34C37), total antibody titers (9, 27, 36), IgG titers (10, 12C14, 16, 18, 22, 28C30, 32, 33), IgM titers (10, 11, 22C24, 32), and/or IgA titers (10, 13, 18, 19, 22, 29, 32). Numerous antigens eliciting the antibody reactions have been also demonstrated to be associated with the disease severity, including antibodies elicited against the spike (S) protein and/or receptor-binding website (RBD) in the S protein (12, 16, 19, 22, 29, 30, 32, 36), antibodies elicited against the nucleocapsid (N) protein (10, 11, 13, 23, 32, 33, 36), and antibodies against both the S and N proteins (14, 24). While the aforementioned observational studies shown positive associations of the antibody titers with the disease severity, several issues still remain to be resolved. For example, since many of these studies did not measure the IgG, IgM, and IgA titers against the S.