Several aspects like failure of antigen presentation or costimulatory signals, defects in the generation of HBsAg-specific CD4 T helper (Th) cells and insufficient production of Th1 and Th2 cytokines upon hepatitis B vaccination have been discussed, but the exact underlying immunological and molecular mechanisms contributing to hepatitis B vaccine non-responsiveness remain largely unclear (10, 19–24). Here, CD4 is linked to hepatitis A virus infection.