Inapparent human infection by JE virus occurred during June–November 1956 in 5% of 404 school children ages 6 to 12 years near the Shinhama and Sagiyama study sites for mosquitoes, birds and pigs. Because human populations were large, this percentage represented a large number of infected children. Infection was independent of the child's age. Children developed antibody during a shorter time interval near Sagiyama (21 August to 19 September) than near Shinhama (17 August to 29 October). Children with pre-existing neutralizing antibody manifested the same JE virus exposure frequency by hemagglutination-inhibition antibody test as children without pre-existing antibody.
Past infection frequencies for JE virus as reflected by neutralizing antibody incidences in June or July 1956 were higher in children 6 to 8 years of age near Sagiyama (.28–.26) than near Shinhama (.08–.16). The percentages of previously infected children cumulated yearly for ages 6 to 12 years at Shinhama, but at Sagiyama past infection frequencies were the same for ages 6, 7 and 8.
Between 1948 and 1956 clinically apparent Japanese encephalitis near Tokyo occurred yearly during August and September. The size of epidemics varied, but correlated roughly with the numbers of serologic diagnoses of JE made yearly at the 406 MGL. No uniform cycle of recurrence was evident, although large epidemics occurred more often in August whereas small epidemics were usually in September.
Present address: The National Institute of Health, Takanawa, Tokyo (M. K., T. O., and T. O.).