Cross reactive memory T cells in a substantial proportion of individuals before exposure to the SARS-CoV-2 virus has been firmly established.54 101–103 The role of cross reactive memory T cells in protection is unclear, however, with some studies linking the presence of low avidity cross reactive CD4+ T cells clones with the severity of covid-19 disease4 52 100 102 108 and others indicating that these cells could facilitate expansion of effective T cell memory to SARS-CoV-2 after vaccination or infection, or both.62 136 144 145. The gene discussed is CD4; the disease is infection.