Abstract

Background: In hepatitis A virus (HAV)-seronegative infants, inactivated hepatitis A vaccines are highly immunogenic. On the contrary, in infants who are HAV-seropositive before vaccination, the interfering effect of passively-transferred maternal anti-HAV antibodies leads to lower post-primary immunization anti-HAV levels, as compared to those achieved by seronegative infants. One possible way to overcome this drawback is to delay hepatitis A vaccination later during the first year of life. The objective of the study was to document the immunogenicity of an inactivated hepatitis A vaccine in 6 months old HAV-seropositive infants, given as two dose regimen consisting of a single primary immunization at 6 months of age, followed by a booster dose 6 months later. Methods: The immunogenicity of one hepatitis A vaccine (Avaxim™ pediatric, Aventis Pasteur) was documented in 108 6 months old, HAV-seropositive infants randomly assigned to receive one priming dose of hepatitis A vaccine either concomitantly with (Group 2) or 2 weeks after the third dose of routine diphteria-tetanus-whole cell pertussis reconstituting lyophilized tetanus conjugated Haemophilus influenzae type b (DTwcP//PRP?T) vaccine and oral poliomyelitis vaccine (OPV) (Group 1). A booster dose was given 6 months later, concomitantly with MMR vaccine. Results: The 91 infants who were HAV-seropositive (ELISA titer >20mIU/ml) at the moment of primo vaccination remained seropositive 1 month later. Geometric mean titers (GMT) decreased from 292 and 278mIU/ml 1 month after the first dose, to 77.6 and 76.0mIU/ml 6 months after, in Groups 1 and 2, respectively. Post-booster titers increased markedly in both groups, with GMTs of 1731 and 1866mIU/ml and geometric mean post/pre-immunization titer ratios of 22.3 and 24.6, respectively. Conclusions: These results suggest that immunological priming induced by a single dose of Avaxim™ pediatric administered to 6 or 6.5 months old, HAV-seropositive infants is present and should not preclude the use of this vaccine in such populations. © 2003 Elsevier Science Ltd. All rights reserved.