A group from Juntendo University Faculty of Medicine has reported seroprevalence of IgG and IgM antibodies in COVID-19 patients, although the cohort size was small including only 34 patients.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8023454/
Using a chemiluminescent microparticle immunoassay (CMIA)-based SARS-CoV-2 IgG test (cat. # 06R90, Abbott),
Severe/Critical cases：within a week after symptom onset＝40％、1～2 weeks＝88%、after two weeks＝100％,
Mild/Moderate cases：within a week after symptom onset＝0％、1～2 weeks＝38%、after two weeks＝100％.
Using an IC IgG antibody assay using the Anti-SARS-CoV-2 Rapid Test (cat. # RTA0203, AutoBio),
Severe/Critical cases：within a week after symptom onset＝60%、1～2 weeks＝63%、after two weeks＝100%,
Mild/Moderate cases：within a week after symptom onset＝17%、1～2 weeks＝63%、after two weeks＝100%.
In this study, IgG titers remained at significantly elevated levels for 2 months, regardless of disease severity. These results indicate that IgG serologic tests could be used as a complementary test to PCR to diagnose COVID-19 from 14 days after symptom onset. However, since this cohort is so small without including asymptomatic individuals, a larger scale cohort is needed to conclude final answer.