The Topic of This Month Vol.17 No.11(No.201)

Influenza, 1995/96 Season, Japan

In the 1995/96 season, influenza virus isolates were mostly type A(H1N1). As was the case in the 1994/95 season in which isolates were mainly type A(H3N2), reports of serious cases of encephalitis and other illnesses were noticeable. In the beginning of the 1996/97 influenza year, IASR has received from three laboratories four reports on isolation of influenza virus type A(H3N2).

Reports of influenza patients during the 1995/96 season, according to the National Epidemiological Surveillance of Infectious Diseases (NESID), started to increase in number in the 48th week (November 26-December 2) of 1995, earlier by three weeks than in the preceding season, reaching the largest number in the 4th week of 1996 (Fig. 1). The total patients encountered during the season from the 35th week of 1995 through the 34th week of 1996 numbered 139.13 per sentinel clinic, which was about half of that during the preceding season (see IASR, Vol. 16, No. 12).

Reports on isolation of influenza viruses in the 1995/96 season totaled 3,735 (Table 1), of which 3,303, being the largest number ever supplied, dealt with isolation of type A(H1N1) at 55 laboratories. In the preceding season, it was isolated very infrequently. Reports on isolation of type A(H3N2) were very few, numbering 407 from 34 laboratories mainly in east Japan and those on that of type B were only 11 from six laboratories.

The weekly reports of influenza virus isolation (Fig. 2) tell that, during the 1995/96 season, type A(H1N1) isolation started in the 45th week (November 5-11) of 1995 and kept up until the 13th week of 1996, with peaks in the 51st week of 1995 and the 4th week of 1996. In the 34th week (August 18-24), the virus was isolated from a patient in Yokohama City. Reports on isolation of type A(H3N2) started later than that of type A(H1N1), formed a small peak in the 9-10th weeks of 1996, and have been continuing even after April. It was isolated in May from patients in an outbreak in Saitama Prefecture (see p. 270 of this issue) and still more in the 29th week (July 14-20) from one in Sendai City and another in Fukushima Prefecture. Type B was isolated from one each in the 51st week of 1995 and the 1st and 5th weeks of 1996, and after April from eight cases during the 15-20th weeks.

Isolation of type A(H1N1) in the 1995/96 season was most frequent from school children in lower grades of elementary schools (at ages of 6-8 years), being 1,232 cases accounting for 38%, followed by children at ages of 3-5 years, being 1,021 cases accounting for 31% (Fig. 3).

Of those with isolation of influenza virus reported to IASR up till now, serious cases of encephalitis, encephalopathy, and cardiovascular disorder are listed in Table 2. Before the 1993/94 season, reports of serious cases were rare. However, during the 1994/95 season, 12 serious cases yielding type A(H3N2) and other three yielding type B were reported. During the 1995/96 season, 10 cases yielding type A(H1N1), one type A(H3N2) and one type B were reported. From one of these patients, type A(H1N1) virus was isolated from cerebrospinal fluid (see IASR, Vol. 17, No. 5). It is noteworthy that there were many serious cases during the two successive seasons, due to type A(H3N2) in the former and type A(H1N1) in the latter season. In addition, besides respiratory illness, there were three hepatitis, one nephritis and two meningitis cases, with type A(H1N1) in the 1995/96 season.

Since the type A(H3N2) viruses isolated in China and other countries after January 1996 had undergone an antigenic change (see IASR Vol. 17, No. 4 and p. 274 of this issue), the type A(H3N2) component of the 1996/97 influenza vaccine strain has been replaced by A/Wuhan/359/95 antigen recommended by WHO (see p. 270 of this issue).

Update for the 1996/97 season as of November 6: Type A(H3N2) virus was isolated from one case on September 4 at Hyogo PHI, two on September 17 at the Sendai National Hospital (see P. 270 of this issue) and one on October 7 at Fukushima PHI.

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