Japanese encephalitis surveillance in China (Province of Taiwan) during 1968-1971. II. Age-secific incidence in connection with Japanese encephalitis vaccination program.

A baseline age/morbidity pattern for Japanese encephalitis (JE) epidemic in China (Province of Taiwan) in the 1960s was sought in JE surveillance. The pattern is characterized by: (1) high incidence in age groups 2-4 years, (2) moderately high incidence in age groups 5-7 years, (3) negligible incidence in children under 1 year of age as well as in adults, and (4) the variability of relative height of peak incidence co-incident with the JE-epidemic size in situ. Concurrently with surveillance, a JE-vaccination program was implemented by the Government. The children of target age groups received two doses of vaccine each during April. Vigilant supervision was given to the campaign to achieve a high coverage rate with two-dose vaccination and meticulous record-keeping. During 1969-1972, 2-year olds were the exclusive target; 52.2% (1969), 71.5% (1970), 74.5% (1971), and 83.0% (1972) of the target age group were covered. A considerable change from the baseline age/morbidity pattern seen in 1968 occurred during the above period. The comparison of epidemicity-adjusted incidence by year indications that incidence reduction in the 2- and 3-year old children in 1971 were substantial and suggests the possible impact of vaccination. The total JE incidence for the period 1968-1971 appears to be much lower in vaccinated population (23 reported and 8 confirmed/0.9 million) than in unvaccinated population (1,330 reported and 679 confirmed/2.2 million).