While a consistent trend and significant reduction of multiple CD99 antigen and polymeric immunoglobulin receptor peptides was associated with severe disease and mortality in critical COVID-19 patients, this distribution was less evident or not observed for polymeric immunoglobulin receptor in the non-ICU population and for CD99 antigen in all cohorts not infected with SARS-CoV-2. Here, CD99 is linked to COVID-19.