A subcutaneously administered, live, high-titre (18,700-60,000 plaque-forming units per dose) varicella zoster virus (VZV) vaccine (zoster vaccine) of the Oka/Merck strain has been evaluated for the prevention of herpes zoster and the reduction of zoster-associated pain in adults aged > or =60 years. Zoster vaccine, when compared with placebo, reduced the burden of herpes zoster illness by 61%, the incidence of herpes zoster by 51% and the incidence of postherpetic neuralgia by 67% during more than 3 years of surveillance. The zoster vaccine caused an initial 1.7-fold rise in VZV antibody titre after 6 weeks that declined progressively over 3 years. Increases in gamma-interferon-secreting peripheral blood mononuclear cells were 2.2-fold greater with the zoster vaccine than with placebo 6 weeks after vaccination. Zoster vaccine was generally well tolerated. The most frequently reported adverse reactions following vaccination were injection-site reactions; the only systemic adverse event with zoster vaccine that differed significantly in incidence from that with placebo was headache.