CD69 and infection: The decrease of the MBC, unswitched/switched MBC and CD69+/CD38+T cells, the differences in Tfh and Th17 cell changes between the elderly population and the middle-aged population, as well as the increase of CD4+/CD8+PD-1+ T cells, CTLA-4+CD4+/CD8+T cells and DNBC, suggest that the elderly exhibit a reduced persistent immune response to BA.5 infection.