Abstract
Synopsis Virosomal hepatitis A vaccine contains formalin-inactivated hepatitis A (strain RG-SB) antigen adsorbed on to immunopotentiating reconstituted influenza virosomes (IRIVs). IRIVs act as an adjuvant and may stimulate both the cell-mediated and humoral immune responses to vaccine antigens. In noncomparative studies, virosomal hepatitis A vaccine (500 radioimmunoassay units) has exhibited immunogenicity in adults and children, inducing seroconversion in almost all volunteers within 4 weeks (usually within 2 weeks) of vaccination. Booster injections of the virosomal vaccine 12 months after the initial dose increased anti-hepatitis A virus (anti-HAV) antibody titres by 11- to 39- fold. Studies of up to 3 years in duration (with a booster vaccination after 1 year) have shown high anti-HAV antibody titres; protection against hepatitis A infection has been estimated to last for 8 to 12.5 years and an unpublished report suggests that 68% of vaccinees may be protected for ⩾30 years. In comparative studies, the virosomal vaccine produced lower anti-HAV antibody titres than an aluminium hydroxide-adsorbed vaccine (strain HM175), but similar percentages of volunteers seroconverted. The virosomal vaccine was shown to have 95% protective efficacy in unpublished preliminary results of a randomised double-blind trial involving Nicaraguan children. The vaccine had better local tolerability than an aluminium hydroxide-adsorbed hepatitis A (strain HM175) vaccine in immunogenicity studies. The most common adverse effect was pain or soreness at the injection site. Thus, virosomal hepatitis A vaccine (strain RG-SB) is highly immunogenic, has demonstrated protection from hepatitis A infection in a preliminary report, and appears to have a relatively low incidence of local adverse effects on injection. The final placement of this vaccine in the prevention of hepatitis A infection will be determined by the duration of protection it offers in comparison to other hepatitis A vaccines and its relative acquisition costs.
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