967 subjects were tested for IgG antibodies against COVID-19. They multiple subgroups of suspected cases, healthy donors, patients with rheumatic disease, and patients positive for polymerase chain reaction. They found PCR and IgG were in agreement in 83% of samples. The levels of IgG and IgM levels were measured and found to not associate with mild and severe disease course.
TL;DR Antibody levels against COVID-19 does not predict clinical severity