Consistent with the hypothesis that boosted Abs are cross-reactive, SARS-CoV-2 S-reactive CD4+ T cells, a prerequisite for class-switched Ab responses, have been detected in the majority of COVID-19 patients and in 34% of uninfected individuals, supporting the existence of shared epitopes between S proteins of endemic CoV and SARS-CoV-2 (68–70). Here, CD4 is linked to COVID-19.