What’s more, preexisting memory CD4+ T cells, getting from infection by the common human coronaviruses, including HCoV-OC43, HCoV-229E, HCoV-NL63, and HCoV-HKU1, could realize cross-reactive with comparable affinity to SARS-CoV-2 (Mateus et al., 2020; Post et al., 2020). Here, CD4 is linked to infection.