The Topic of This Month Vol.18 No.5(No.207)

Pertussis, Japan, 1982-1996

Pertussis epidemics recurred every 4 years or so in Japan; cases and deaths were rapidly decreasing until postvaccination fatal incidents occurring during 1974-1975. Due to the temporary interruption of vaccination in 1975 and the subsequent decline in the vaccination rate, pertussis epidemics reemerged in the whole country during 1976-81. In 1981, use of a new pertussis vaccine (an acellular pertussis vaccine called "adsorbed purified pertussis vaccine") comprising detoxified pertussis toxin (PT) and filamentous hemagglutinin (FHA) antigens as the main constituents with curtailed side effects was started and the vaccination rate was restored (Kimura, M., Dev. Biol. Stand., 73, 5-9, 1991), resulting again in decrease in number of cases notified (Fig. 1). After 1975, however, the primary immunization with pertussis vaccine in mass vaccination was given at ages over 2 years. In December 1988, the Ministry of Health and Welfare (MHW) issued a notification that the basic vaccination policy against pertussis should be individual inoculation and, even in mass vaccination, the primary immunization may be started at the age of 3 months. In 1994, the Preventive Vaccination Law was amended and the inoculation policy of adsorbed diphtheria-tetanus-acellular pertussis (DTaP) combined vaccine was changed vastly from mandatory mass inoculation to individual inoculation upon recommendation and the age of 3-12 months was recommended for primary series of DTaP inoculation.

The trend of weekly reports of pertussis cases to the National Epidemiological Surveillance of Infectious Diseases (NESID) is shown in Fig. 2. During 1982-83, peaks are seen in April-May and August-September. As cases decreased in number, such distinct peaks faded, but in 1986, 1990 and 1991, when there were relatively a large number of patients, small peaks turned up but in August-September only (Fig. 2). The cases per annum were 12.59 per sentinel clinic in 1982 (reports from all sentinel clinics totaled 23,675), then gave a gradually decreasing pattern with an increase every 4 years or so, and were the fewest, 1.51 (3,666), in 1993. The trend is similar to that of the patients notified as seen in Fig. 1.

The cases per annum reported to NESID were about 20 times as many as those notified in compliance with the Communicable Disease Prevention Law. If extrapolated to the whole country, there must be tens of thousands of cases every year even now. As shown in Fig. 3, the majority of pertussis patients are infants, and the age group of under one year accounts for half of all patients. The ratio tended to become high in the years when there were relatively many cases. In recent years, the ratio of cases aged over 10 years tended to rise slightly.

Reports per annum on isolation of Bordetella pertussis in IASR show a trend similar to those of patients(Fig. 4). Isolation of the organisms was reported from prefectural and municipal public health institutes and general clinical institutions in 31 prefectures and six cities, so it may be estimated that B. pertussis is distributed widely all around the country. IASR does not collect information of the organisms by the serotype, but according to the nationwide surveys made during 1988-92 by Kimura et al. (J. Jpn. Assoc. Infect. Dis., 70, 19-28, 1996), the types involving factors 1 and 3 were predominant (98%).

According to the National Epidemiological Surveillance of Vaccine-Preventable Diseases, the ratio of those possessing the ELISA antibody against the pertussis protective antigens (PT and FHA) in the serum of healthy population was significantly higher in the vaccinated population than in the unvaccinated one in every age group (Fig. 5). The ratios of antibody positives were compared in 1990, 1994, and 1995 (Fig. 6). Both the prevalence of anti-PT and anti-FHA rose year to year in the age group of under 2 years. For example, the ratio of those possessing anti-PT antibody rose from 17% in 1990 to 39% in 1994 and further to 55% in 1995. These results may reflect the heightened vaccination rate among infants aged under one year (5.6% in 1990, 23% in 1994, and 33% in 1995 received more than two doses for primary immunization) in compliance with the 1988 notification of MHW and the 1994 amendment of the Preventive Vaccination Law.

Among pertussis patients reported to NESID during the decade from 1987 to 1996, those with histories of vaccination accounted for 1.3% (918/68,586), and Kimura et al. (J. Jpn. Assoc. Infect. Dis., 70, 19-28, 1996) reported that among pertussis patients from which B. pertussis was isolated, those with histories of two or more doses of the primary series of vaccination accounted for only 1.8%; therefore the most reliable measure of pertussis prevention in Japan should be vaccination of infants. Continuous future investigations seem warranted as to how the 1994 amendment of the Preventive Vaccination Law would affect the vaccination rate, the antibody prevalence, and the incidence of pertussis.

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