Immunogenicity Study of an Inactivated Hepatitis A Vaccine in Infants and Young Children

Background: Infants with passively-transferred maternal antibody (PMA) to hepatitis A virus (HAV) have a blunted immune response to hepatitis A vaccine. We compared the immunogenicity of hepatitis A vaccine among infants with and without PMA, vaccinated on different schedules. Methods: Infants were randomized to one of three groups, each receiving two doses of 720 EL.U. of hepatitis A vaccine (HAVRIX, Glaxo SmithKline) according to the following schedules: Group 1 at ages 6 and 12 months; Group 2 at ages 12 and 18 months; Group 3 at ages 15 and 21 months. We determined antibody to HAV (anti-HAV) status of mothers at the time of delivery, and measured infants' anti-HAV concentrations at the time of the first vaccine dose (baseline), and at 1, 7 and 12 months thereafter. Anti-HAV concentrations \> 33 milli-International Units/milliliter (mIU/mL) were considered protective. We monitored adverse reactions using diary cards and chart reviews. Results: A total of 239 infants were enrolled, including 134 born to anti-HAV negative mothers (Groups 1N, 2N, 3N) and 105 born to anti-HAV positive mothers (Groups 1P, 2P, 3P). At month 12, 6 months after the second vaccine dose, the difference in GMC between Groups 1P and 1N was the only statistically significant difference within groups (p\<0.05). There were no statistically significant differences in GMC among groups of infants born to anti-HAV negative mothers ("N" groups), but the difference between Group 1P and Group 3P infants was significant (p \< 0.05). No serious adverse reactions related to vaccination were detected. Conclusions: Hepatitis A vaccine is immunogenic among infants born to anti-HAV negative mothers, and among those born to anti-HAV positive mothers and vaccinated beginning as young as 12 months old. The persistence of PMA for at least six months among the majority of infants born to anti-HAV positive mothers results in lower seroconversion rates and GMC's.