When comparing the spike-specific Ab, CD4+ and CD8+ responses of participants with PCR-confirmed infections or positive M/N reactivity (termed “INF+VACC,” known as “hybrid immunity”15) prior to or after the boost (termed “INF+VACC 3x”) to other participants, a significant increase was observed in both T cell subsets from hybrid-immune participants prior to the boost (Figure 3C; INF+VACC vs. VACC in pre-3). Here, CD4 is linked to infection.