Unlike early HBV infection in mild CVID, i.e., the development of a chronic process, the risk of severe allergic reactions is much higher when the HBV infection occurs after IVIG initiation [3] (containing not only anti-HBs, but also anti-HBc IgG); moreover, C4 dynamics are more visible here (Figure 1). Here, C4A is linked to common variable immunodeficiency.