Immunogenicity and safety of a new inactivated hepatitis A vaccine in Thai young adults.

ABSTRACT

In view of the increasing median age of hepatitis A virus (HAV) infection observed recently in Asia, and the resulting increased number of symptomatic cases occurring in adults, with the concomitant risk of outbreaks, immunization against this agent on a national scale might be considered. An open clinical trial was conducted in Thai adolescents and young adults in order to establish the immunogenicity and safety of a new inactivated hepatitis A vaccine. At 24-week intervals, two doses (primary dose and booster) of the hepatitis A vaccine (160 antigenic units per dose) were administered to 80 HAV-seronegative healthy volunteers, their ages ranging from 16 to 25 years. Local and systemic reactions were recorded within the first 7 days after each injection. Anti-hepatitis A virus antibody concentrations were measured by a modified radioimmunoassay before and one month after each injection. No serious adverse reactions were reported. Local reactions were confined to transient pain at the injection site, occurring within 24 hours after injection in 42.5% of the subjects after the first dose and 24.1% of the patients after the booster dose. Systemic reactions (particularly asthenia or myalgia) were observed in 35.0% and 8.9% of subjects after the first and the booster injection, respectively. Most of these reactions were transient. One month after the first dose, all 78 formerly seronegative subjects had attained satisfactory seroconversion levels of anti-HAV antibody concentrations (> or = 20 mIU/ml) which they maintained until the booster. The booster dose elicited a 21-fold increase of HAV antibody levels, with a geometric mean titer of 2,964 mIU/ml (95% CI, 2,467-3,560), indicative of long-term protection. This new inactivated hepatitis A vaccine appears to be safe and highly immunogenic upon administration of a primary dose followed by a booster dose after 24 weeks. In countries where socio-economic improvement has postponed hepatitis A infection from early childhood (mostly asymptomatic) towards adolescence and adulthood, with the symptoms increasing in severity, inclusion of inactivated hepatitis A vaccine in a preventive vaccination program might be of benefit.