Dose range evaluation of a new inactivated hepatitis A vaccine administered as a single dose followed by a booster.

A total of 242 healthy adults were immunised with a first dose of an investigational inactivated hepatitis A vaccine. Three concentrations (3, 6 and 12 EU [ELISA units]) of the experimental vaccine were used and compared to a licensed reference vaccine. The aim was to determine the antigenic concentration of the study vaccine inducing the highest seroconversion rate and anti-Hepatitis A virus (HAV) antibody response at 2 weeks after the primary immunisation. A booster dose was given at month 6. At 15 days after the primary immunisation the seroconversion rates in subjects vaccinated with the 6 and 12 EU vaccines were 78 and 94%, respectively. At 30 and 180 days after the primary immunisation the percentages of seropositivity were 100% for both groups. The antibody response to the 12 EU study vaccine was similar to that to the reference vaccine. The percentages of seropositivity at 15 and 180 days after the primary immunisation were 94 vs 93%, and 100 vs 93% in the experimental and reference vaccine respectively. Thus, because it induces early and lasting seroconversion, the 12 EU study vaccine seems to be the most effective as a high potency HAV vaccine.