Dose response study of an inactivated hepatitis A virus vaccine.

In this dose-response study evaluating 3 different doses and 2 time schedules, the highest dose, 25 units of viral protein antigen, induced a seroconversion antibody response against hepatitis A virus (anti-HAV) over 10 mIU in all vaccinees by week 3 after one dose, indicating rapid onset of protective antibody levels. Following the second dose, given 4 weeks later, rising titers were observed for 20 weeks, when the third and final dose of 25 units was given. The GMT of anti-HAV at 24 weeks (before the third vaccine injection) was 398 mIU/ml for the 25-unit dose, compared to 42 and 65 mIU/ml, respectively, for the 12.5 unit and 6.25 unit doses. The third vaccine dose at 24 weeks gave a booster response in all vaccinees, but the increase in titers was most pronounced in the 25 unit group, which had reached a GMT of 6593 mIU/ml when tested 4 weeks later. Side-effects included mainly local reactions and a few cases of mild diarrhoea, and did not differ for the 3 doses studied. In Scandinavia, hepatitis A vaccines will probably be used mainly for vaccination of foreign travellers. This category usually requires a rapid immunization schedule with few doses. Two doses approximately 3-4 weeks apart will probably be accepted by most travellers, and a single dose may provide short-term protection for most individuals. The need and timing for a booster dose will have to be further studied.