Immunization against hepatitis A in the first year of life: Priming despite the presence of maternal antibody

Background. Maternal antibodies interfere with hepatitis A vaccination in y oung infants. We examined the response to a high dose hepatitis A vaccine a dministered concomitantly with a combination of diphtheria-tetanus toxoids- acellular pertussis-inactivated poliovirus vaccine/Haemophilus influenzae t ype b vaccine to initially seropositive vs. initially seronegative infants. Methods. Three hundred subjects were originall;y planned to be enrolled at age 6 to 10 weeks and received hepatitis A vaccine (formalin-inactivated va ccine, SB-Bio, 720 enzyme-linked immunosorbent assay units) at 2, 4 and 6 m onths concomitantly with a diphtheria-tetanus toxoidsacellular pertussis-in activated poliovirus vaccine/H. influenzae type b vaccine. Children initial ly seropositive received a booster dose at 12 months of age. An additional 100 twelve-month old infants previously not vaccinated with hepatitis A vac cine were given 1 dose, to observe the primary response at that age. Reacto genicity was recorded on diary cards for the 3 subsequent days. Immunogenic ity was measured at Months 2, 4, 5, 10 and 11 after administration of the f irst vaccine dose. For the subjects enrolled at 12 months, blood was drawn before and 1 month after the first vaccination. Results. Of 297 initially enrolled infants 36% were seronegative before vac cination (Group A). The geometric mean concentration (GMC) (milli-Internati onal Units/ml) of the seropositive infants (Group B) before immunization wa s 2587. The GMCs of Group A infants 1 month after each dose and at 12 month s of age were 93, 518, 1656 and 786, respectively. For Group B infants, the respective GMCs were 1165, 460, 508 and 167. One hundred subjects of Group B received a booster dose at age 12 months; at Month 13 all were seroposit ive with a GMC of 1902. For comparison, a third group of 100 not previously immunized 12-month-old infants (Group C) were enrolled and received 1 dose of hepatitis A vaccine with pre- and postimmunization GMCs of 52 and 120, respectively. Conclusions. Our results suggest that the initially seropositive infants we re primed despite maternal antibody interference. The hepatitis A vaccine w as well-tolerated in this population of young infants.