A recent study looked at the immune reaction in newborn infants up to the age of one year who had received the HepB vaccine to see if their immune reaction differed from adults getting the same vaccine. What they found was that the infant, even after age one year, did react differently. Their antibody levels were substantially higher than adults (3-fold higher) and it remained higher throughout the study.
In essence, they found that the babies responded to the vaccine by having an intense Th2 response that persisted long after it should have disappeared, a completely abnormal response.