The Topic of This Month Vol.21 No.12(No.250)

Influenza, 1999/2000 season, Japan
(IASR 2000; 21: 260-261)

From 1987, roughly 2,500 sentinel clinics of pediatricians (and partly of general physicians) participating in the National Epidemiological Surveillance of Infectious Diseases (NESID) reported weekly influenza-like illness cases. At the same time of the enactment of the Infectious Diseases Control Law in April 1999, about 5,000 influenza sentinel clinics were designated; 3,000 of pediatricians for covering child cases and newly nominated 2,000 of general physicians for adult cases. The 1999/2000 season was the first to be followed under the new influenza-surveillance system.

Incidence of clinically diagnosed influenza patients: Weekly incidence of influenza during the 13 seasons from 1987/88 to 1999/2000 seasons is shown in Fig. 1. In 1999/2000 season, earlier than usual by two or three weeks, epidemics started to spread in the beginning of December 1999, and patients rapidly increased in number all over the country to a peak during the fourth to fifth weeks of 2000 (Fig. 2). Subsequently, patients rapidly decreased to less than one per sentinel clinic in the 12th week.

Under the new reporting system after April 1999, the ages of patients between 30 to over 80 years, which used to be placed under the same label of over 30 years, are divided at 10-year intervals due to the increased sentinel clinics of general physicians (Table 1). The ages of influenza patients reported during the first quarter of 2000 (from the 1st to the 13th week) were compared with those during the same period of the preceding year. No difference in the percentage of patients of any age group can be seen between 0 to 9 years, but a decrease in the group of 10-14 years of age (Fig. 3). The number of patients over 30 years of age was on the increase, not so markedly though.

Isolation of influenza viruses: The weekly reports of influenza virus isolation at prefectural and municipal public health institutes (PHIs) in the whole country during the past five seasons are shown in Fig. 4. The 1999/2000 season was characterized by a mixed epidemic with type A (H3N2), having prevailed continually for four seasons, and type A (H1N1) after being absence for three seasons. Both type A viruses begun to prevail and reached the peaks at nearly the same time. The reports of type A (H1N1) isolation outnumbered those of type A (H3N2) at a ratio of 6 to 4. It was the first time that the two types were isolated so frequently following the start of the Infectious Agents Surveillance in 1982 (Table 2 and IASR, Vol. 17, No. 11). Type B was seldom isolated unlike the preceding season. Both type A (H1N1) and (H3N2) were isolated in most prefectures, although a disparity in the number of reports can also be seen. Type B was sporadically isolated in nine prefectures.

Patients of known ages from which influenza virus was isolated are summarized in Fig. 5. The patients from which type A (H3N2) was isolated formed a peak at one year of age and were fewer than those in the preceding season in any age group. Type A (H1N1)-isolated patients were the largest in number in the group of 7 years of age, and outnumbered type A (H3N2)-isolated ones in all age groups from 3 years to 10-14 years.

In hemagglutination-inhibition (HI) tests, 93% of type A (H3N2) isolates in 1999/2000 season were closely related antigenically to a 1999/2000 vaccine strain, A/Sydney/5/97. In addition, a small number of antigenic variants similar to A/Panama/2007/99, a 2000/01 vaccine strain, were occasionally isolated. Forty-seven percent of type A (H1N1) isolates were closely related to a 1999/2000 vaccine strain, A/Beijing/262/95. Isolates similar to A/New Caledonia/20/99, a 2000/01 vaccine strain, also showed increase (see p. 262 of this issue and IASR, Vol. 21, No. 10).

The excess mortality: The excess fatal cases among the total deaths (by any cause) counted at about 8,000 in 1999/2000 season, which may correspond to about 1/4 that of the preceding season (see p. 265 of this issue).

Encephalitis/Encephalopathy: It has recently become apparent that many cases of acute encephalitis/encephalopathy occur among infants and children in influenza-epidemic seasons. Nationwide surveillance made by a study group of the Ministry of Health and Welfare (headed by Dr. T. Morishima, Professor of Nagoya University) during January 1 to March 31, 1999 revealed 238 cases, of which 217 were applicable to the criteria of encephalitis/encephalopathy defined by the study group. During January 1 to March 31, 2000, 142 cases were reported and 109 of them were applicable. The reports dealing with isolation/detection of influenza viruses from pharyngeal specimens or cerebrospinal fluids of acute encephalitis/encephalopathy patients at PHIs counted at 65 in 1999/2000 season, including 34 type A (H3N2) and 31 type A (H1N1) viruses.

Antibody prevalence and virus isolation in 2000/01 season: The National Epidemiological Surveillance of Vaccine-Preventable Diseases performed in the autumn of 2000 prior to 2000/01 season (preliminary results from 13 prefectures) found the influenza HI antibody prevalence rate (HI titer of 40 or higher) among healthy individuals against A/New Calendonia/20/99 (H1N1) was 20-40% in younger ages, and lower among adults. The rate against A/Panama/2007/99(H3N2) was high in the age group of 0-14 years, and lower in adults, especially senior citizens. The rate against B/Yamanashi/166/98 was low among all age groups but that of 10-14 years (see p. 267 of this issue).

A type A (H3N2) virus was isolated in Hiroshima Prefecture on September 4, 2000 (see p. 268 of this issue). A type A (H1) virus was detected in Yokohama City on September 25. Three type A (H3) viruses were detected in Aichi Prefecture during late October. The information is updated whenever necessary in IDSC, NIID homepage (

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