The Topic of This Month Vol.19 No.12(No.226)

Influenza, 1997/98 Season, Japan

The following article deals with influenza in Japan during the 1997/98 season based on the reports submitted to the Infectious Disease Surveillance Center (IDSC), the National Institute of Infectious Diseases. It specifically outlines the following three topics:

1) Incidence of clinically diagnosed influenza patients reported by pediatricians and physicians serving at about 2,500 sentinel clinics throughout Japan.
2) Isolation of influenza viruses reported by approximately 60 prefectural and municipal public health institutes (PHIs) in Japan.
3) Incidence of acute encephalitis/encephalopathy associated with influenza.

Weekly incidence of influenza-like illness during the past 11 seasons, provided to the National Epidemiological Surveillance of Infectious Diseases (NESID) was delineated and is shown in Fig. 1a. Although few cases of influenza were reported in November and December 1997 in the 1997/98 season, cases suddenly increased in the 3rd week of 1998, exceeding 50 cases/sentinel clinic in 29 different prefectures in the 5th week. A total of 136,929 patients were reported by all sentinel clinics throughout Japan. Since the beginning of influenza surveillance in 1987, this is the largest number of cases ever reported. The number of cases began to decrease from the 7th week and the influenza epidemic ceased altogether in the 10th to 12th week. Recent reports of Influenza epidemics throughout the end of November or early December to the end of the year in Japan are all small-scale incidents. Large-scale influenza epidemics are reported throughout the months of January to March, and the number of reports declines in April and May. The 1997/98 season in Japan was characterized by large-scale epidemics in a short period, January and February of 1998 (Fig. 2).

The age distribution of clinically diagnosed influenza patients reported during the first quarter of 1998 (from the 1st to 13th week) (Fig. 3a) shows about the same number of patients of any age between one and nine years compared with one-third to one-quarter number of patients less than one year old. Currently, children 10 to 14 years of age are categorized in the same age group graded by five years and patients of over 15 years of age are rarely reported because many of the sentinel clinics specialize in pediatrics. Therefore, details of these age groups are not known.

The reports of virus isolation are summarized.(table 1 & Fig. 4) In addition to isolation from specimens collected at the above-mentioned sentinel clinics, the data include isolation from those taken from outbreaks that occurred in kindergartens and primary and junior high schools. The 1997/98 season had the largest number of influenza virus isolates ever reported, totaling 6,144. This outnumbered the 1996/97 season where 6,072 isolates were reported. The majority of isolates (98%) were type A (H3N2). There were 12 type A (H1N1) isolates during the 2nd to 8th week of 1998 and 114 type B isolates during the 2nd to 26th weeks (late June). After incidence of influenza decreased, small-scale outbreak of type B influenza occurred in a primary school in Saitama Prefecture and in a junior high school in Shizuoka Prefecture in May and in a primary school in Sendai City, Miyagi Prefecture in June (see IASR Vol. 19, Nos. 7, 8 and 9).

Some type A (H3N2) isolates were sent from PHIs to the Department of Virology I, National Institute of Infectious Diseases for antigen analysis. The number of A/Wuhan/359/95-like strains (the vaccine strain for the 1997/98 season) was found to be about the same as that of A/Saga/128/97-like variant strains (A/Sydney/5/97-like strain, having provoked epidemics in many countries of the world) (see p. 274 of this issue).

Recently, reports of severe encephalitis/encephalopathy complicating influenza among children have increased (see the Topic of IASR, Vol. 18. No. 12). Fig. 1b illustrates the number of cases of encephalitis/encephalopathy admitted to about 500 sentinel hospitals of NESID throughout Japan (although encephalitis, encephalopathy, Reye syndrome, and myelitis were searched for, most cases reported were acute encephalitis/encephalopathy). These cases were not correlated with clinically diagnosed influenza patients before the 1996/97 season (Fig. 1a). A simultaneous increase of clinically diagnosed influenza patients and of acute encephalitis/encephalopathy ones was clearly noted in January and February 1998. Although the diagnoses of these illnesses were not based on isolation of the etiological agents, an increase in acute encephalitis/encephalopathy patients was revealed during the 1997/98 influenza season.

In the 1997/98 season, influenza virus was isolated or influenza virus genome was detected by RT-PCR at PHIs in 74 cases of acute encephalitis/encephalopathy (including one Reye syndrome case). This outnumbered 19 such cases in the 1996/97 season and was the largest number ever reported (Fig. 1c). The influenza virus was isolated from cerebrospinal fluid of 13 cases, and from lung/bronchia of another, and, in addition, a positive result was obtained only by RT-PCR from cerebrospinal fluid of another case. In the remaining 59 cases, the virus was isolated from nasopharynx (see IASR, Vol. 19, Nos. 4, 6, and 7 for the reports from PHIs). Forty-three cases (58%) were aged 1-3 years and those aged 5 or older were few (Fig. 3c). At the time of reporting, deaths were confirmed in 11 of the 74 cases. The Influenza and Encephalitis/Encephalopathy Study Group of the Ministry of Health and Welfare estimated the total number of fatal cases of influenza encephalitis/encephalopathy in the 1997/98 season at 100-200.

Concerning the vaccine strains for the 1998/99 season in Japan, three strains, A/Beijing/262/95 (H1N1), A/Sydney/5/97 (H3N2), and B/Mie/1/93, were selected on the basis of antigen analyses of the isolates during the 1997/98 season (see p. 275 of this issue). Influenza vaccine was once administered to people ranging from kindergartens to junior high school students principally by mass vaccination. However, the Preventive Vaccination Law was amended in 1994 and since then, vaccination of all people, particularly from kindergartners to junior high school students and the elderly, has been made on voluntary individual immunization basis. In Europe and North America, vaccination against influenza of the elderly is actively recommended because influenza often accompanies pneumonia and other complications that can lead to death (CDC, MMWR, 47, No. 38, 1998; CDSC, CDR, 8, No. 34, 1998).

Update: Under the National Epidemiological Surveillance of Vaccine-Preventable Diseases Program, determination of influenza antibodies is being undertaken by the PHIs on serum samples taken from healthy individuals recruited in the autumn of 1998. According to a preliminary report of analyses still under conduct at IDSC, the positive rate of anti- A/Sydney/5/97 (H3N2) was relatively high in young and low in elderly people, and those to A/Beijing/262/95 (H1N1) and B/Beijing/243/97 (variant strain) were generally low (see p. 275 of this issue).

At the end of September, for the first time in the 1998/99 season, a type A (H3N2) strain, resembling the A/Sydney/5/97 strain, was isolated in Fukuoka City. In November, type A (H1N1) in Ishikawa Prefecture (see p.275 of this isse) and type A(H3N2) in Kanagawa Prefecture and type B in Shizuoka Prefecture (see p.275 of this issue). Information on the Internet provided by IDSC shows the incidence of influenza in different districts in Japan (URL: since November 1997. Therefore, information concerning influenza during an epidemic can be quickly accessed.

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