CD4 and COVID-19: With our in vitro study design, we could detect the highest percentage of IFN‐γ+ CD4+ T cells in the COVID‐19 group (median 0.35% IFN‐γ+ of CD4+ T cells) and the lowest in participants who had only received two vaccination doses (median 0.091% IFN‐γ+ of CD4+ T cells).