CTLA4 and susceptibility to pneumonia measurement: Specifically, we identified a minimal immune signature consisting of three parameters: a high percentage of activated CD4+ T-lymphocytes, a low percentage of CTLA-4+ T-lymphocytes, and a low percentage of immature B-lymphocytes, that were strongly associated with COVID-19 severity expressed as the need for hospitalisation, pneumonia, impaired lung functions, and a persistent dyspnoea four to eight weeks after the COVID-19 diagnostic PCR test.