Previous studies showed that CD4-positive lymphocyte count in peripheral blood correlated significantly with the acquired antibody titer in patients treated with anti-CD20 antibodies, suggesting that CD4-positive T cells established by COVID-19 vaccination are necessary for the generation of high-affinity antibodies and development of memory B-cells and may contribute to long-term memory immunity against SARS-CoV-2 [30, 40–44]. Here, CD4 is linked to COVID-19.