The Topic of This Month Vol.26 No.11(No.309)

2004/05 influenza season, Japan

(IASR 2005; 26 : 287-288)

During the 2004/05 season (September 2004-August 2005), approximately 1,500,000 cases of influenza were reported by sentinel clinics throughout Japan, the largest epidemic in the past 11 seasons, with an estimated total of approximately 17,700,000 cases occurring nationwide. The epidemic was a mixture of influenza B, AH3, and AH1 viruses; the majority was influenza B viruses, most belonging to the Yamagata lineage, a characteristic only observed in Japan.

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 nationwide (3,000 - pediatric, 2,000 - internal medicine). Compared to the previous 10 seasons, the peak number of cases per sentinel per week in the 2004/05 season was the third largest after the 1994/95 and 1997/98 seasons (see However, because the epidemic period was prolonged, the total number of cases per sentinel during 2004/05 season was 321.5, the largest figure in the past 11 seasons. In 2005, the rise in weekly case counts occurred later than usual, with nationwide activity increasing rapidly after sentinel reporting exceeded 1.0 during week 3. Activity peaked during week 9, then rapidly decreased to less than 1.0 per sentinel during week 19 (Fig. 1). At the prefectural/district level (Fig. 2), early increases in sentinel reporting were seen in the Kanto, Tokai and Hokuriku districts, while late increases were observed in the Hokkaido and Tohoku districts. Although the number of cases per sentinel decreased to less than 0.1 after the nationwide epidemic ended, during weeks 27-30, cases per sentinel exceeded 0.1 for the first time since 1990, reflecting the regional epidemic seen in Okinawa Prefecture (see IASR 26:243-244, 2005). By the national reporting for all cases of hacute encephalitish initiated in November 2003, 51 cases of influenza encephalopathy were reported during the 2004/05 season (see p. 295 of this issue).

Excess mortality due to influenza: Excess mortality due to influenza during the 2004/05 season, based on the total number of deaths in Japan, was estimated at 15,100 during February-April 2005 (see p. 293 of this issue).

Isolation of influenza viruses: During the 2004/05 season, prefectural and municipal public health institutes (PHIs) nationwide reported a total of 3,348 influenza B virus isolates, 2,513 influenza AH3 isolates, and 184 influenza AH1 isolates (reports as of October 21, 2005, Table 1).

Influenza AH3 viruses were initially isolated in Aichi and Osaka Prefectures during weeks 36 and 39, of 2004, respectively (see IASR 25:290-291, 2004), and after continuously being isolated in small numbers in many districts, increased sharply during weeks 3-5 in 2005 (Fig. 1). Although the number of isolates decreased after week 6, influenza AH3 viruses were continuously isolated without interruption in Okinawa and other prefectures during the summer. In Nara Prefecture, an outbreak was reported in a facility during July-August (see IASR 26:244-245, 2005). Influenza B viruses were isolated in Yamagata Prefecture during week 42 and in Ibaraki and Hyogo Prefectures during week 46, and increased from week 3 of 2005 simultaneously with influenza AH3 virus. Activity peaked late during week 7 and continued until week 20 (Fig. 1). In contrast, although influenza AH1 virus was rarely isolated during the 2002/03 (1 case) and 2003/04 (5 cases) seasons, it was reported in Fukushima and Okayama Prefectures during week 46 of 2004, and continuously isolated from week 48 of 2004 until week 13 of 2005. Influenza AH1 viruses were subsequently isolated during weeks 17, 18, and 19 (1 case each) and weeks 25 and 28 (2 cases each) in different districts.

Influenza AH3 viruses were frequently isolated from cases 12 years of age or younger, while infrequently isolated from those 13 years or older. In contrast, influenza B viruses were isolated with greatest frequency in children 6 years of age among the pediatric population, and individuals in their 30s among adults. Influenza AH1 viruses were mainly isolated from children 2-10 years of age (Fig. 3).

Antigenic characteristics of 2004/05 isolates and the vaccine strains for 2005/06 season: The antigenicity of influenza AH1 virus was similar to that of A/New Caledonia/20/99, the vaccine strain for the 2004/05 season. Of the influenza AH3 viruses, A/Fujian/411/2002-like strain (the representative strain being A/Wyoming/3/2003, a component of the 2004/05 vaccine), the primary circulating strain from the previous season, was frequently isolated during the first half of the season, while A/California/7/2004-like strains constituted the majority of isolates during the latter half of the season. Of the influenza B viruses, 99% was the Yamagata lineage, with most resembling the B/Shanghai/361/2002 strain, a component of the 2004/05 vaccine (see p. 289 of this issue).

Vaccine strains for the 2005/06 season are as follows: A/New York/55/2004 (influenza AH3 virus), an A/California/7/2004-like strain which replaces A/Wyoming/3/2003; A/New Caledonia/20/99 (influenza AH1 virus); and B/Shanghai/361/2002 (influenza B virus), which belongs to the Yamagata lineage, were continuously selected (see IASR 26:270-272, 2005).

Production of influenza vaccine and influenza vaccine coverage among the elderly: For the 2004/05 season, 20,740,000 vaccine doses were produced and 16,430,000 doses used. The demand for the 2005/06 season has been estimated at approximately 21,500,000 doses (see p. 300 of this issue). The vaccine coverage rate among the elderly (primarily those 65 years of age or older), in compliance with the Preventive Vaccination Law, was 47% for the 2004/05 season (Blood and Blood Products Division, Pharmaceutical and Food Safety Bureau, Ministry of Health, Labour and Welfare).

Avian influenza outbreaks: In late June 2005, outbreaks due to low pathogenic A/H5N2 virus occurred among birds in Ibaraki (see p. 298 of this issue) and Saitama Prefectures. Approximately 2,000,000 domestic fowl either died of infection or were slaughtered. No cases of apparent human A/H5N2 infection have been identified in Japan or other countries.

Preparedness for the next influenza pandemic: WHO has issued a global influenza pandemic preparedness plan (see The Ministry of Health, Labour, and Welfare has also stepped up pandemic preparedness efforts in Japan by establishing a national taskforce for influenza preparedness planning on October 28, 2005, and is in the process of developing a national influenza action plan.

Because pathogen diagnosis is important in differentiating severe acute respiratory syndrome from influenza, laboratory reports of influenza virus detection from overseas travelers during non-epidemic periods have been increasing (see IASR 24:281-282, 2003 & 25:278-279, 2004). During non-epidemic periods in 2004/05, influenza AH3 virus was isolated from returning travelers from Vietnam (see IASR 26:222, 2005), China (see IASR 26:243, 2005), and Thailand (see p. 303 of this issue) suspected of contracting avian influenza. On the other hand, isolation of influenza AH3 (see p. 302 of this issue) and AH1 viruses from cases with no history of foreign travel received attention during non-epidemic periods in the latter half of the 2004/05 season. The importance of year-round influenza surveillance has become much clearer, while improvements in laboratory testing systems at public health institutes, as well as cooperation from health care providers with specimen collection, are both critically needed.

Preliminary reports for 2005/06 season: Influenza AH3 virus was isolated in the following prefectures or cities: Mie Prefecture (weeks 36 and 37, see p. 303 of this issue), Nagasaki Prefecture (week 37, primary school outbreak), Kobe City (week 39, a returning traveler from Thailand, see p. 303 of this issue), Okinawa Prefecture (week 42, junior high school outbreak), and Kobe City (week 43). In addition, influenza A/H1N1 was detected by PCR in Tokyo (week 36, primary school outbreak).

On November 7, designated as kickoff day, efforts have begun toward developing an integrated strategy against influenza for this upcoming season (see p. 301 of this issue).

Return to the TopPage

Return to the IASR HomePage
Return to the IASR HomePage(Japanese)

Back to Home