Encephalitis caused by Japanese encephalitis virus occurs in annual epidemics throughout Asia, making it the principal cause of epidemic
viral encephalitis in the world. No currently available
vaccine has demonstrated efficacy in preventing this disease in a controlled trial. We performed a placebo-controlled, blinded, randomized trial in a northern Thai province, with two doses of monovalent (Nakayama strain) or bivalent (Nakayama plus Beijing strains) inactivated, purified
Japanese encephalitis vaccine made from whole virus derived from mouse brain. We examined the effect of these
vaccines on the incidence and severity of
Japanese encephalitis and
dengue hemorrhagic fever, a disease caused by a closely related flavivirus. Between November 1984 and March 1985, 65,224 children received two doses of monovalent
Japanese encephalitis vaccine (n = 21,628), bivalent
Japanese encephalitis vaccine (n = 22,080), or
tetanus toxoid placebo (n = 21,516), with only minor side effects. The cumulative attack rate for
encephalitis due to Japanese encephalitis virus was 51 per 100,000 in the placebo group and 5 per 100,000 in each
vaccine group. The efficacy in both
vaccine groups combined was 91 percent (95 percent confidence interval, 70 to 97 percent). Attack rates for
dengue hemorrhagic fever declined, but not significantly. The severity of cases of
dengue was also reduced. We conclude that two doses of inactivated
Japanese encephalitis vaccine, either monovalent or bivalent, protect against
encephalitis due to Japanese encephalitis virus and may have a limited beneficial effect on the severity of
dengue hemorrhagic fever.