IFNG and COVID-19: Notably, all close contacts developed responses at lower frequencies than 4%, while 64 (71.11%) and 32 (35.56%) of the 90 COVID-19 patients developed marked responses at the frequencies of higher than 4% for IFNγ+CD4+ T cells (Fig. 1a) and IFNγ+CD8+ T cells (Fig. 1b), respectively.