However, our study based on a sample of 184 patients has several limitations as it revealed prior infections by HCoV and infections by SARS-CoV-2 but did not explore underlying mechanisms: (i) the T-CD4+ and T-CD8+ cellular responses were not studied, (ii) despite providing evidence of past HCoV infection, the delay between HCoV infection and SARS-CoV-2 infection was unknown and (iii) the quality of the HCoV antibodies in terms of, for instance, their neutralisation potential was not known. The gene discussed is CD4; the disease is infection.