When COVID-19 induces viral infections, the B cells induced by follicular helper CD4+ T cells could produce antibodies capable of neutralizing the pathogen and can combine with cytolytic CD8+ T cells to kill pathogen-infected cells, which leads to a decrease in peripheral blood lymphocytes, while the COVID-19 vaccine could induce robust lymphocyte generation, which led to elevated peripheral blood lymphocytes. This evidence concerns the gene CD4 and viral infectious disease.