A group from School of Medicine, Keio Univ. has reported a cohort study on antibody responses in asymptomatic to mild new coronavirus (COVID-19) patients who were positive for PCR testing in Japan.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7787511/
Many overseas cohort studies have targeted patients with more symptomatic and severe cases, and antibody responses have appeared in 80% to 100% of patients three weeks after infection. Numerous studies have also shown that these antibodies correlate with age, severity, lymphopenia, and serum CRP levels.
This cohort study was conducted in asymptomatic to milder patients than in known cohort studies, and 87.5% of asymptomatic patients and 23.5% of mild patients did not have antibody responses.