Several in silico studies reported that certain pathogenic bacteria, including those that cause diphtheria, tetanus, and pertussis, could generate cross-reactive immunity to SARS-CoV-2 because these bacteria and SARS-CoV-2 share multiple epitopes, including numerous epitopes recognized by CD8+ and CD4+ T cells, generating broad protection coverage [41,73]. This evidence concerns the gene CD4 and diphtheria.