In a study investigating antibody profiles in patients from Atlanta, Georgia, researchers measured immunoglobulin G (IgG) and immunoglobulin M (IgM) levels over time in 28 patients hospitalized for severe COVID-19, 15 patients with mild COVID-19, control participants, and 116 individuals with illnesses in March and April 2020. To accomplish this, the researchers developed and validated three distinct assays: one targeting the receptor-binding protein of the spike protein subunit S1 to measure IgG levels, another targeting the whole S1 protein to measure IgM levels, and a third targeting an envelope protein to also measure IgM levels.