Current status of measles in Japan
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Approximately 20 000 to 30 000 cases of measles are reported annually in Japan, although the actual number of measles infections is considered to be five to ten times higher than the number of reported cases. Despite the availability of effective and safe live attenuated vaccines, regional outbreaks in Okinawa, transmission in adults, and secondary vaccine failures continue. Recent advances in molecular technology have contributed to molecular epidemiological studies, new concepts of asymptomatic infection, and the identification of different characteristics among measles virus genotypes. Measles virus strains isolated in Japan since 1984 were classified into the genotypes C1 (–1985), D3 (1985–1990), D5 (1990–1997), and Chicago-type D3 (1997–1999) from the results of sequencing the hemagglutinin gene. After 2000, the D5 genotype emerged, and, recently, the H1 genotype, which is now dominant, was introduced from Korea. Some of the currently circulating wild-types have different characteristics (high growth rate at 39°C–40°C). The cumulative vaccine coverage has reached 81%, but most measles cases involved unvaccinated individuals. Measles is a preventable disease and can be eradicated by increasing the vaccine coverage, and promoting the motivation for vaccination, in accordance with the worldwide measles strategy.
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