CD4 and infection: This study shows that 1) This proliferative assay is useful to discriminate between past infection patients and control individuals, with 5 CD4+ blasts/μL of blood being the best cut-off to discriminate both groups; 2) It is also useful for distinguishing seronegative from seropositive PCRpos patients, since the percentage of patients with positive SARS-CoV-2 T-cell response was significantly lower in the seronegative group, thus suggesting that impaired proliferation of this T-cell subpopulation and the lack of specific IgG might be related.