Oka/Merck Varicella Vaccine

Abstract

Synopsis Oka/Merck varicella vaccine is a live attenuated vaccine produced from the Oka strain of the herpesvirus varicella zoster virus (VZV). The Oka strain of VZV is recognised as having low virulence while eliciting humoral and cell-mediated responses to VZV. In numerous studies, Oka/Merck varicella vaccine achieved a seroconversion rate of >95% in healthy children and adolescents. In immunocompromised children with leukaemia in remission, seroconversion was achieved in 80% of vaccinees 12 months after vaccination. Cell-mediated immune responses generally closely paralleled humoral immune responses. The vaccine is also immunogenic in healthy adults. In healthy children, a high level of protection was conferred by Oka/Merck varicella vaccine, and in the only double-blind placebo-controlled study conducted to date the vaccine was 98% effective in protecting healthy children against varicella during 2 varicella seasons. In children with leukaemia, Oka/Merck varicella vaccine was 80% effective in protecting against mild varicella and conferred 100% protection against moderate and severe varicella. In healthy adults, the rate of protection against varicella was reported to be approximately 50%. All observed cases of varicella in healthy and immunocompromised vaccinees were mild and there were no reports of any disease-related complications. Interestingly, in children and adolescents with leukaemia, the incidence of herpes zoster was lower in children who received Oka/Merck varicella vaccine than in similar children who had natural immunity. Mild varicelliform rash (occasionally accompanied by fever) has been observed in healthy children, adolescents and adults who have received Oka/Merck varicella vaccine. Pain at the injection site was the adverse effect most frequently reported in one large study of the vaccine in healthy children. In children with leukaemia, vaccine-associated rash has been reported in up to 50% of vaccinees. When Oka/Merck varicella vaccine was administered to healthy children in combination with measles, mumps and rubella vaccine there was no increase in the incidence of skin rashes compared with that caused by measles, mumps and rubella vaccine in combination with placebo. Thus, Oka/Merck varicella vaccine is a highly immunogenic vaccine that confers a high level of protection against typical varicella in healthy children and adolescents, and in children and adolescents with leukaemia. While results of further long term studies investigating the persistence of protective immunity against varicella are awaited with interest, available data indicate that Oka/Merck varicella vaccine is a major advance in the prevention of this common and highly infectious disease.