Study suggests T-cells don't determine survival in severe COVID-19 The coronavirus disease (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), causes mild to moderate illness in the general population. Some people, however, may experience severe symptoms, which can be potentially fatal. Severe COVID-19 has been tied to T cell lymphopenia, but no causal effect of T cell deficiency on disease severity has been established. Researchers at the Laboratory of Persistent Viral Diseases, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health found that T cell-depleted groups of rhesus macaques developed virus-neutralizing antibody responses and also class-switched to immunoglobulin G (IgG).