In COVID-19 patients infected with the new coronavirus (SARS-CoV-2), IgG antibody tests of (Architect and iFlash) were used to measure serum-IgG in mild and severe cases. In some mild cases, IgG was not detected. However, when a method of testing for a neutralizing antibody was used, it was really detected from such mild cases in which IgG was not detected. So, this means that we must be careful in choosing IgG assay for COVID-19.
IgG rises up on the average 11th day (range of 7 to 20 days) after symptoms appear in severe patients, while on average the 22nd day (range of 14 to 79 days) for mild patients, and the total amount of IgG produced was higher in severe patients than in the mild.
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0241104