Decreased seroconversion rates might occur among preterm infants with birth weights less than 2,000 grams after administration of HepB at birth. However, by the chronological age of 1 month, all preterm infants, regardless of initial birth weight, are likely to respond as adequately as larger infants. Infants who weigh less than 2,000 grams born to HBsAg-positive mothers and mothers with unknown HBsAg status (if the mother’s HBsAg status cannot be determined within 12 hours of birth) must receive immunoprophylaxis with HepB and hepatitis B immune globulin (HBIG) within 12 hours of birth. The initial vaccine dose should not be counted toward completion of the hepatitis B series, and 3 additional doses of HepB should be administered, beginning when the infant is age 1 month. Infants weighing less than 2,000 grams born to HBsAg-negative mothers should receive the first dose of the HepB series at hospital discharge or at chronological age 1 month (even if weight is still less than 2,000 grams), whichever comes first.