In our patients with humoral immunodeficiencies and no breakthrough COVID-19, 4 out of 5 patients in our study had a detectable SNM-specific IFN-γ+ CD4+ T cell response after just 2 doses of CoronaVac; the single patient who did not have a detectable IFN-γ+ CD4+ T cell response had an IL-2+ CD4+ T cell response. Here, CD4 is linked to COVID-19.