CD40LG and COVID-19: Therefore, we measured IgA in the sera instead of in secretion because our previous study showed that paired bio-sampling of saliva and blood in patients with COVID-19 had strong positive correlations with SARS-CoV-2 Spike (S) and receptor-binding domain (RBD)-specific immunoglobulin (IgM, IgG, and IgA) levels [21].