The Topic of This Month Vol.22 No.11(No.261)

Measles, Japan, 1999-2001

(IASR 2001; 22: 273-274)

In Japan, measles vaccine has been dosed by the routine immunization system since October 1978, resulting in marked decrease in the cases and deaths (Fig. 1). The vaccine coverage investigated by Isomura, S. and his associates of the Research Group on Immunization of the Ministry of Health, Labour and Welfare (MHLW) accounted for 81% in the year 2000, exceeding 80% for the first time (75% in the year 1996, see IASR Vol. 20, No. 2, 1999). The ages for routine immunization under the Preventive Vaccination Law are 12 to up to 90 months. For the standard schedule of measles vaccination, however, an earliest possible immunization in 12 to 24 months of age is recommended (those who are over the ages of routine immunization may receive voluntary immunization as an option). Since the last issue of the topic on measles (see IASR Vol. 20, No. 2, 1999), cases have again been on the increase during 2000-2001. The following is an analysis of the epidemiological data of 1999-2001, to contribute further to the promotion of measles control.

The National Epidemiological Surveillance of Infectious Diseases: The Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law) was enacted on April 1, 1999. Measles was listed as one of the category IV infectious diseases. In addition to the conventional report by the pediatric sentinel clinics (the sentinel clinics had increased from about 2,000 to 3,000), it was started to report also adult measles cases (over 18 years olds, mostly inpatients) by about 500 sentinel hospitals.

The number of measles patients reported by the pediatric sentinel clinics after the enactment of the Infectious Diseases Control Law in April 1999 are shown in Fig. 2. The patients reported in 1999 (cumulative total of the 14th-52nd weeks) were the fewest, 2.04 per pediatric sentinel clinic. They increased again from 2000, and reached 10.95 per sentinel (the cumulative number of cases being 32,890) as of the 41st week of 2001, 1.4 times more than 7.57 per sentinel for the whole year of 2000 and the largest in number in the past 7 years. When looked at by prefecture (Fig. 3), the patients reported in 2001 are more than 20.0 per pediatric sentinel clinic in nine prefectures and more than 4.0 per sentinel hospital in six prefectures. The increase in number of patients as compared with 1999 and 2000 is clearly shown (see IASR, Vol. 22, No.5 for Kochi and No.7 for Ishikawa). On the other hand, the number of cases per pediatric sentinel clinic reported by four prefectures was as small as less than 2.5. It is a recent characteristic tendency that the prefectures with many patients and those with few patients are located side by side (see IASR Vol. 20, No. 2, 1999). Of the ages of patients (Fig. 4), one year of age was predominant in the reports by the pediatric sentinel clinics, followed by those of 6-11 months and 2 years of age; as a whole, half of the patients were younger than 2 years. By the reports from sentinel hospitals dealing with adult measles, those at the ages of 20-24 years were predominant, followed by those of 15-19 years and 25-29 years. The age groups in which patients increased in 2001 compared with 2000 are those over 10 years (1.7 times more) reported by pediatric sentinel clinics and those aged 20-39 years (2.3 times more) reported by sentinel hospitals.

Isolation of measles viruses: Because of dissemination of B95a cells, which are highly sensitive to measles viruses, the viruses can be isolated more easily and epidemic strains are being monitored by a world-wide laboratory network (see IASR, Vol. 20, No. 2, 1999). The reports of measles virus isolation by prefectural and municipal public health institutes (PHIs) are submitted to the Infectious Disease Surveillance Center, National Institute of Infectious Diseases (NIID). Before October 25, 2001, the reports counted at 16 in 1999, 88 in 2000, and 86 in 2001, totaling 190, from a total of 54 blood and 145 nasopharyngeal specimens (reports of isolation from both the specimens are included). The nucleotide sequence analysis conducted at the Department of Viral Diseases and Vaccine Control, NIID shows that the genotype of measles viruses isolated in 2001 are type D5 in most districts of Japan, whereas all viruses isolated in Okinawa are D3. Type H1 prevailing in China and Korea has been isolated in Kawasaki and Tokyo (see p. 278 of this issue).

The National Epidemiological Surveillance of Vaccine-Preventable Diseases (see p. 275 of this issue): The investigation in 2000 found a low vaccine coverage for one year of age. The antibody negatives (the blank area in Fig. 5: susceptibles) by the gelatin particle agglutination (PA) test ('positive' means titer higher than or equal to 1:16) were 48% of one year of age and 21% to two years of age. Of those aged 5-6 years, born after cessation of measles-mumps-rubella (MMR) live-virus vaccine, there were more susceptibles (8.8%) than among those aged 3-4 years (6.7%). Among the vaccinees, the antibody-positive rate was extremely high, being 99%. The geometric mean titer (GMT) was kept at a high level from young to aged groups. From the results of this investigation and the population by age found by the national census taken on October 1, 2000, about one million of those aged under 2 years are estimated to be susceptibles to measles.

Action plans to control measles and the future problems: WHO set a sight of measles morbidity and mortality reduction as the expanded program for immunization (EPI) after polio eradication (see p. 286 of this issue). In Japan, the deaths officially comprehended through the Vital Statistics of Japan are still counting two digits (see Fig. 1 and the material on p. 288). The measles epidemics in Japan at the moment are characterized by insufficient suppression from a low vaccination rate. As a result of this, we are faced with the following two peculiar trends that have never been experienced; 1) Epidemics of small and medium sizes are always occurring somewhere, showing regional differences, and 2) Unvaccinated one-year children are mainly infected, but adult measles is also on the increase. Under such circumstances, increasing vaccine coverage is a must for the time being. It is necessary to immunize infants as soon as possible after the one-year birthday, according to the present vaccination formula. In the area where measles is suppressed by the increased vaccination, the chances of exposure to wild-type viruses reduce, resulting in 1) the growing up of non-vaccinees and vaccinees having failed to acquire immunity, remaining susceptible, and 2) the reducing of the immunity level of those who have once acquired immunity by vaccination because of no chance of natural boosting. As a result, measles among adults, particularly pregnant women, and among newborn babies become important. Therefore, care must be taken in imported infection from the epidemic areas not only in foreign countries but also within the country and acquiring infection when traveling to epidemic areas. In future, surveillance should be enhanced 1) to check vaccination histories of measles cases, 2) to monitor the people's immune status in each prefecture or city by seroepidemiological studies (see p. 279 of this issue), and 3) to analyze the antigen structures and nucleotide sequences of the prevailing measles viruses.

Some districts such as Hokkaido, Osaka, Kochi, and Okinawa have launched to cope with measles epidemics by whole the local government personnel (see p. 279-285 of this issue). The Japan Pediatric Society, the Japanese Society of Child Health, and the Japan Pediatric Association have submitted a request to the MHLW and the Ministry of Education, Culture, Sports, Science and Technology for improving vaccine coverage and measles control (see p. 285 of this issue). At the Technical Advisory Group Meeting held at the WHO Regional Office for the Western Pacific in Manila in August 2001, measles control was adopted as an agenda and it is going to be a whole Asian activity.

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