Measles virus is classified into clades A-H, including 23 genotypes. In Japan, D3 and D5 have mainly been isolated; H1 was often isolated during 2002-2003 and D5 during 2006-2007 (see p. 244&245 of this issue and http://idsc.nih.go.jp/iasr/measles-e.html).
According to WHO, about 20 million people develop measles every year in the world, and about 345,000 deaths due to measles were estimated in 2005. The Americas and Korea accomplished measles elimination in 2000 and 2006, respectively (see p. 262 of this issue). The Western Pacific Region including Japan set 2012 as the target date for measles elimination (see p. 261 of this issue).
The National Epidemiological Surveillance of infectious Diseases (NESID): In compliance with the Infectious Diseases Control Law, the measles cases reported in 2006 from about 3,000 pediatric sentinel clinics were the fewest in the past, and adult measles cases (including ≥18 years until March 2006 and ≥15 years from April 2006) reported from about 450 sentinel hospitals largely decreased during 2005-2006. In 2007, however, having peaked on week 21 (Fig. 1), cumulative reported cases per pediatric sentinel during weeks 1-31 was 0.77 (cumulative reported cases being 2,307, including 1,283 males and 1,024 females), and reported cases per sentinel hospital was 1.69 (cumulative reported cases 772, including 414 males and 358 females). Adult measles cases are particularly increasing.
In 2007, cases have increased on a national scale principally boosting up the reports in Kanto district (Fig. 2). Number of prefectures with cases more than 0.5 per sentinel during weeks 1-31 increased to 16 for measles and 28 for adult measles. There is a tendency that more cases have occurred in Eastern Japan.
Measles cases of the ages of 10-14 years reported from pediatric sentinel clinics in 2007 (Fig. 3) are markedly increasing. Age distribution of cases (Fig. 4) shows that those of ≥10 years have increased in 2007, accounting for 44%. The number of 0-year cases is as usual, while that of 1-4 years, which used to account for 40-50%, has largely decreased to 22%. From sentinel hospitals, on the other hand, relatively severe cases of adult measles cases have been reported. Adult measles cases in 2007 have mostly been of 20-24 years (Fig. 3), and those younger than 30 years accounted for 77%.
Acute encephalitis is a category V notifiable infectious disease under the NESID. During weeks 13-33 in 2007, 8 cases of measles encephalitis have been reported (see Table 1 and p. 256 of this issue).
School outbreaks: In 2007, school closures (completely or partially) have occurred successively in many places of the country. School outbreaks of measles accompanying temporary closing of class work reported to the Ministry of Health, Labour and Welfare from April 1 through July 21 counted at 263, particularly high schools and universities/colleges counted at 73 and 83, respectively (http://idsc.nih.go.jp/disease/measles/pdf/meas070727.pdf).
The National Epidemiological Surveillance of Vaccine-Preventable Diseases (see Fig. 5 and p. 241 of this issue): Measles antibody positives with the gelatin particle agglutination (PA) test (antibody titer 1:16≤) were 68% of 1-year children in the 2006 survey; a 24-point increase over the 2001 survey, however the rate did not attain a goal higher than 95%. The measles antibody positives of 2-year children increased to 94% and such high rate was maintained by those ≤8 years. In the age groups from 9 years through 19 years, there were antibody negatives in about 10%, who played a key role of the 2007 epidemic. There remained a small number of antibody negatives among those ≥40 years.
Characteristics of recent measles epidemics in Japan: Having experienced the 2001 measles epidemic involving an estimate of 278,000 cases on a national scale, mainly among infants and young children (see IASR 25: 60-61, 2004), measles control has been intensified all over the country. In particular, measles vaccination rate of target age group for routine immunization (1-7.5 years) increased. From April 1, 2006, measles-rubella vaccine has been introduced for routine immunization in compliance with the Preventive Vaccination Law (see IASR 27: 85-86, 2006) and a 2-dose schedule has been adopted from June 2, 2006, but the vaccine coverage in the second immunization (5-6 years, within one year before primary school entrance) differed largely among municipalities, the national average being 80% (see p. 259 of this issue).
In the spring of 2006, in the southern part of Ibaraki (see p. 251 of this issue) and Chiba Prefectures (see IASR 27: 226-228, 2006), local epidemics of measles occurred. In November 2006, high school students in Tokyo and Saitama Prefectures developed measles during school excursion to Okinawa and four of them were hospitalized (see IASR 28: 145-147, 2007). From the end of 2006, reported cases increased in Tokyo and Saitama Prefectures. Then, epidemics spread to Chiba and Kanagawa Prefectures and further to the whole country during the “Golden Week” holidays in May 2007. This epidemic has not yet been controlled as of September 2007. Besides, Japanese high school students developing measles during traveling in the country where measles elimination has been achieved and foreigners developing the disease after traveling to Japan have been reported; the episodes attract attention as an international problem.
Cases of 10-20s of age have markedly increased during the 2007 epidemic, including both non-vaccinees and persons who have received one-dose measles vaccination (see p. 245, 247, 249 and 253 of this issue). Together with the increase in adult measles cases, newborn infant measles cases (see IASR 28: 195-196, 2007) and a 60-year-old case suspected of symptomatic reinfection have also been reported (see p. 258 of this issue).
Future countermeasures against measles in Japan: Aiming at enhancement of immunity of the generation who were damaged by the 2007 epidemic intensively, the second immunization of the first grade students of junior high schools and those aged 17-18 years (including third grade students of high schools) with measles-rubella vaccine are scheduled from 2008 as a temporary measure for five years under the Preventive Vaccination Law (see p. 260 of this issue). From now on, measles control at schools (see p. 247-250 of this issue) and control at each district (see p. 245 & 251-255 of this issue) with close cooperation with the Ministry of Education, Culture, Sports, Science and Technology, will be important.
To accomplish elimination of measles by 2012, active measles control measures seem important; such as establishing a quick evaluation system of immunization practice to maintain 2-dose vaccination coverage of at least 95%, and changing the present sentinel surveillance to mandatory reporting of all measles cases including vaccine history to allow quick response at the time of occurrence of the initial case (see p. 255 of this issue). If cases further decrease approaching elimination, laboratory diagnoses of all cases as far as possible are desirable (see p. 245 of this issue and IASR 28: 221-223, 2007).