If pre-pandemic donors respond to peptide stimulation, the percentage of responding CD4+ T-cells is always <0.1%.5 Authors of these papers speculate that these–mainly CD4+–SARS-CoV-2-reactive T-cells are probably induced by past infection with one of the endemic “common cold” coronaviruses (HCoVs), which share at least partial sequence homology with SARS-CoV-2. Here, CD4 is linked to infection.