CD4 and COVID-19: More importantly, the analysis of the virus-specific immune effectors in the 7 M (Fig. 6c–e) among non-reinfected individuals revealed that even though the XBB Nab titers reduced to closely comparable levels across the groups, individuals who had recovered from severe COVID-19 still exhibited significantly higher IFNγ+CD4+ T cell responses (P < 0.05) and trendily higher IFNγ+CD8+ T cell responses (P = 0.196) than mild convalescents (Supplementary Table 5).