CD40LG and agammaglobulinemia: This may be due the high prevalence of more severe antibody deficiency in the COV-AD cohort; amongst patients with CVID enrolled in COV-AD, 88% were deficient in IgA (median concentration 0.2 g/L), 76% deficient in IgM (median concentration 0.3 g/L) and 67% deficient in both IgA and IgM as defined by the criteria used within the UK PIN registry paper.