Current status of measles in Japan: molecular and seroepidemiological studies

Abstract

On account of the measles vaccination campaign, with vaccinations carried out on the first birthdays of children, the number of reported cases of measles was reduced to 545 in 2005, which is the lowest so far in Japan. We conducted a molecular epidemiological study of measles virus to determine the circulating measles virus genotypes in Japan since 1984. Different genotypes, C1, D3, D5, and H1, were the major strains isolated in outbreaks in 1984, 1987–1988, 1991–1993, and 2000, respectively. When measles was in the control phase, a sporadic outbreak was reported, but the causative virus was found to be of imported measles virus lineage. We also conducted a seroepidemiological study to investigate the persistence of vaccine-acquired immunity in Himeji City, Japan. Before 1990, vaccine coverage was 84.5% and it increased gradually, to 88.5% in 1991–1995, 92.7% in 1996–2000, and 94.6% after 2000. Measles outbreaks were observed annually before 1978 and in 1980, 1981, 1984, 1990, and 1996; there were no measles cases after 1997 in Himeji City. In 1994–1998, a serological study of 795 sera showed that measles neutralization test (NT) antibodies were sufficiently preserved, even 12 years after the first-dose immunization. In 1999–2003, 26 (3.7%) of 695 sera were negative for NT. The positive rate for measles NT decreased to approximately 90% as the elapsed time after the first-dose immunization increased to 6 or 7 years. The immunity obtained after receiving measles vaccine decays by 6–7 years after the first dose when the measles was controlled. A two-dose schedule of measles vaccine was implemented in Japan in 2006; we should continue molecular and serological surveillance.