Dependence on the birth season of the level of haemagglutination inhibition antibodies against Japanese encephalitis virus in the Japanese population.

According to the frequency of haemagglutination inhibition (HI) antibodies against Japanese encephalitis (JE) virus the Japanese population born during the years of JE epidemic falls into 2 groups. The population born during the epidemic season of JE (from July to October) showed a lower rate of HI antibody positivity to JE virus than those born during the non-epidemic season (from November to June). This phenomenon was not related to the birth place of investigated subjects and was not related to JE vaccination.