To confirm the functional capacity of cTfh cells from COVID-19 patients to help B-cell differentiation and antibody production, and compare the functional cTfh cell capacity in severe and mild COVID-19 patients, we isolated cTfh and non-cTfh (CD3+ CD4+ CD45RA− CXCR5−) cells from four severe and mild COVID-19 patient samples from the acute stage of disease as well as from 4 healthy controls (Supplementary Tables 4 and 5). Here, CXCR5 is linked to COVID-19.