Why are vaccines generally not given to infants younger than 6 weeks of age in the U.S.?

Mainly because little safety or efficacy data exist on doses given before 6 weeks of age, and the vaccines aren’t licensed for this use. The data that exist suggest that the response to doses given before 6 weeks is poor and, in the case of Haemophilus influenzae type b (Hib) vaccine, the response could be detrimental to the infant by possibly reducing the immune response to subsequent doses of Hib conjugate vaccine. Hepatitis B vaccine is an exception because infants respond adequately to this vaccine as early as the day of birth and receipt of this vaccine at birth is necessary to protect infants born to HBsAg-positive mothers.