Single dose inactivated hepatitis A vaccine: rationale and clinical assessment of the safety and immunogenicity.

In comparison with the classical immunisation schedules (0-1-6 or 0-1-12 months) for hepatitis A, a 0- and 12- or a 0- and 6-month schedule would have important advantages by reducing the number of injections and discomfort and increasing scheduling convenience and patient compliance. It would be convenient if a single dose with enough antigen could protect both rapidly and for at least 12 months, when the booster dose would be given. Several clinical trials have been carried out with an inactivated hepatitis A vaccine containing 1,440 EL.U. (1 ml), according to a 0-12 and a 0-6 vaccination schedule. This hepatitis A vaccine is safe and well tolerated. It offers a rapid seroresponse: 14 days after a single dose the seroconversion is 88% (95% C.I.: 84.6-90.9). The 0-12 schedule study showed good persistence of hepatitis A virus (HAV) antibodies with a seroconversion rate of almost 95% at month 12. Booster doses given at 6 or 12 months result in a substantial rise in antibody levels; according to these antibody titres, the 1,440 EL.U. vaccine can be expected to confer comparable duration of protection as the 720 EL.U. vaccine, i.e., 10-20 years. Preliminary data show that timing of the booster may not be critical for the antibody response. In conclusion, the 1,440 EL.U. hepatitis A vaccine is safe, offers rapid seroconversion, and is highly immunogenic. The persistence of HAV antibodies until month 12 allows a certain flexibility in the administration of the booster: month 6 or 12, and a 0-12 or 0-6 schedule can increase the vaccination compliance.