Background: Coronavirus Disease 2019 (COVID-19) causes severe acute respiratory failure. Antibody-dependent enhancement (ADE) is known as the mechanism for severe forms of other coronavirus diseases. The clinical progression of COVID-19 before and after IgG antibody seroconversion was investigated. Methods: Fifty-three patients with reverse transcriptase PCR (RT-PCT)-confirmed COVID-19 viral pneumonia with or without respiratory failure were retrospectively investigated. The timing of the first IgG antibody against SARS-CoV-2-positive date, as well as changes of C-reactive protein (CRP) as an inflammatory marker and blood lymphocyte numbers, was assessed using serial preserved blood samples. Findings: Ten patients recovered without oxygen therapy (mild/moderate group), 32 patients had hypoxemia and recovered with antiviral drugs (severe/non-ICU group), and 11 patients had severe respiratory failure and were treated in the ICU (6 of them died; critical/ICU group). The first IgG-positive date (day 0) was observed from 5 to 18 days from the onset of disease. At day 0, a CRP peak was observed in the severe and critical groups, whereas there was no synchronized CRP peak on day 0 in the mild/moderate group. In the severe/non-ICU group, the blood lymphocyte number increased (P=0.0007) and CRP decreased (P=0.0007) after day 0, whereas CRP did not decrease and the blood lymphocyte number further decreased (P=0.0370) in the critical/ICU group. Interpretation: The respiratory failure due to COVID-19 viral pneumonia observed in week 2 may be related to an antibody-related mechanism rather than uncontrolled viral replication. In the critical form of COVID-19, inflammation was sustained after IgG seroconversion.