Incidence of influenza: Under the National Epidemiological Surveillance of Infectious Diseases, clinically diagnosed influenza cases have been reported weekly by approximately 5,000 influenza sentinel clinics (3,000 of pediatrics and 2,000 of internal medicine). The total number of cases per sentinel in the whole 2005/06 season was 204.6, being the medium-sized epidemic or the 5th largest epidemic during the recent 10 seasons. Weekly cases per sentinel exceeded 1.0 in the nationwide level during week 50 of 2005, which further increased during the year-end and the beginning of the forthcoming year. The number of cases peaked at 32.4 in week 4 of 2006, and decreased to less than 1.0 in week 14. In week 17, the number of cases exceeded 1.0 again and then decreased gradually (Fig. 1). The height of the peak was low, only the 7th high during the recent 10 seasons (http://idsc.nih.go.jp/idwr/kanja/weeklygraph/01flu.html).
Cases per sentinel increased early in western Japan and late in eastern Japan. In Okinawa Prefecture, unseasonable epidemics occurred during weeks 21-29, as was the case in the 2004/05 season (see p. 304 of this issue). Also in Hokkaido, such unseasonable epidemics as never seen before occurred during weeks 20-24 (see p. 305 of this issue).
By the national reporting for all cases of "acute encephalitis", 51 cases of influenza encephalopathy were reported (see p. 307 of this issue).
Excess mortality due to influenza: According to the estimation by the NIID model based on the total number of deaths in Japan (IASR 24: 288-289, 2003), the excess mortality due to influenza during the 2005/06 season was estimated at 6,849 (occurring during November 2005-January 2006), which was the 7th largest number during the recent 10 years.
Isolation of influenza viruses: During the 2005/06 season, prefectural and municipal public health institutes (PHIs) isolated 3,400 influenza AH3 viruses, 1,336 influenza AH1, and 514 influenza B (reports as of October 27, 2006, Table 1).
Weekly isolation and isolation by prefecture are shown in Fig. 1 and Fig. 2 (see p. 295), respectively. Influenza AH3 viruses were isolated in week 36 of 2005 in Mie Prefecture (IASR 26:303-304, 2005) and an outbreak was reported at a primary school in week 37 in Nagasaki Prefecture (IASR 27:11, 2006). Influenza AH3 virus isolation gradually increased after week 43 of 2005, peaked in week 3 of 2006, then decreased although reports continued until week 21. Influenza AH1 viruses were isolated in week 36 of 2005 in Tokyo, and with a local epidemic in Aichi Prefecture in week 46 (IASR 27: 12, 2006) as a start, influenza AH1 virus isolation increased, then decreased in weeks 10-18 in 2006, but a small number of reports from different districts continued until week 32. Influenza B viruses were isolated in Osaka and Kanagawa Prefectures in week 50 of 2005 (IASR 27:12-13, 2006), and few reports continued until a small peak in week 11 of 2006 and then decreased. From week 15, influenza B virus isolation increased again, peaked in week 21 in many prefectures (IASR 27:150-153, 2006) such as Okinawa and Hokkaido with local epidemic, and then continued without interruption until week 31.
The age distribution of influenza virus-isolated cases shows that influenza AH3 viruses were isolated at similar frequencies from cases of 1 to 12 years of age from more cases than the 2004/05 season except 8, 9, and 10 years. Above 15 years, with a peak at 30s, there were more cases of every year group than the 2004/05 season. While influenza AH1 virus isolation, 7 year-cases were the peak and those younger than 9 years accounted for approximately 90%. Influenza B viruses were isolated in cases aged 12-19 years, which were rather rare in the 2004/05 season (Fig. 3).
Antigenic characteristics of 2005/06 isolates and the vaccine strains for the 2006/07 season: The majority of influenza AH1 viruses was similar to A/New Caledonia/20/99, the vaccine strain for the 2005/06 season. Of influenza AH3 viruses, A/Wisconsin/67/2005-like strains, which showed a difference in antigenicity from A/California/7/2004-like strains (the representative strain being A/New York/55/2004, a component of the 2005/06 vaccine) were frequently isolated. Influenza B virus strains belonging to the B/Yamagata lineage, being prevalent in the two preceding seasons, were not isolated: all were B/Malaysia/2506/2004-like, which belong to B/Victoria lineage (see p. 295 of this issue).
For the 2006/07 season, the vaccine strains of influenza AH3 and B viruses were replaced to A/Hiroshima/52/2005, an A/Wisconsin/67/2005-like strain, and B/Malaysia/2506/2004 of Victoria lineage respectively, and of influenza AH1 viruses, A/New Caledonia/20/99 was continuously selected (IASR 27:267-268, 2006).
Production of influenza vaccine and influenza vaccine coverage rate among the elderly: In the 2005/06 season, 20,820,000 vaccine doses were produced and 19,320,000 doses used. According to the survey of influenza vaccine coverage, a demand of approximately 21,500,000-22,800,000 doses is expected for the 2006/07 season. The vaccine coverage rate among the elderly (primarily those 65 years of age or older), in compliance with the Preventive Vaccination Law, was 52% in the 2005/06 season (Blood and Blood Products Division, Pharmaceutical and Food Safety Bureau, Ministry of Health, Labour and Welfare, http://www.mhlw.go.jp/shingi/2006/06/s0614-5.html).
Preparedness for the next influenza pandemic: Current phase of alert in the WHO global influenza preparedness plan is 3 (Pandemic alert: New virus causes human cases but no or very limited human-to-human transmission). Making for provision for increased human-to-human transmission, influenza (H5N1) has been specified as a designated infectious disease according to the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law) by the Japanese government ordinance on June 12, 2006 (see p. 309 of this issue) and guidelines has been prepared by the experts council of the Next Influenza Pandemic (http://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou04/).
Conclusion: In influenza virus surveillance, acquiring isolates is important for virus analysis as the basic data to select vaccine strains. However, more and more materials after used at medical facilities for rapid diagnostic kits are brought to PHIs, which disturbs virus isolation. Since virus isolation will be impossible from the specimen put into the suspension fluid used for kits, the medical professions are requested for understanding and cooperation to collect another specimen for virus isolation and to submit it to PHIs.
On the other hand, under such circumstance that occurrence of highly pathogenic avian influenza is still going on in foreign countries (see p. 311 of this issue) and emerging of the new influenza is under strict watch (see p. 312 of this issue), reports of detection of influenza viruses during summer season, which used to be considered non-epidemic periods in Japan (Fig. 1), and those from cases who developed influenza after overseas traveling are on the increase (Table 1). Year-round influenza surveillance is becoming more important, and further enrichment of the laboratory testing systems at the PHIs is necessary.
Preliminary reports for 2006/07 season (http://idsc.nih.go.jp/iasr/prompt/graph-ke.html): As of November 7, 2006, one influenza B virus was isolated in week 36 in Toyama Prefecture from a child returning from Thailand and one influenza AH3 virus was detected by PCR in Hyogo Prefecture in week 37 from an adult returning from the Philippines. Influenza B viruses were isolated in week 38 in Hiroshima and Shiga Prefectures and a small local epidemic was reported in Hiroshima Prefecture (IASR 27: 268-269, 2006). In week 39, influenza AH1 viruses were isolated in Osaka Prefecture from children infected from their mother traveling to Hong Kong (see p. 316 of this issue) and in Okayama Prefecture from children infected from their father traveling to China.
On November 10 as the kickoff day, MHLW has started the action according to the integrated strategy against influenza for this upcoming season (see p. 315 of this issue).