The Topic of This Month Vol.25 No.11(No.297)

2003/04 influenza season, Japan

(IASR 2004; 25 : 278-279)

During the 2003/04 season, approximately 790,000 cases were reported by influenza sentinel clinics throughout Japan, with an estimated total of 9,230,000 cases occurring nationwide, activity of medium-sized proportions compared to the previous ten seasons. Influenza AH3 viruses were most frequently isolated, 90% of which were A/Fujian/411/2002-like strains. Influenza B viruses were rarely isolated, with B/Shanghai/361/2002, belonging to the Yamagata lineage, accounting for approximately 80% of all B isolates. Influenza AH1 viruses were infrequently seen.

In early 2004, outbreaks of influenza A(H5N1) viruses occurred among birds mainly in Asia; human infections, including fatal cases, were also identified in Thailand and Vietnam. Although person-to-person infection has not been identified, vigilance against the emergence of new subtypes of influenza A virus has been heightened (see p. 287 of this issue).

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. The height of the peak in 2003/04 ranks sixth among the past 10 seasons. In 2003/04, the total number of cases reported per sentinel was 166.0, the fourth lowest figure following 65.6 in 2000/01, 138.2 in 1995/96, and 143.7 in 2001/02. During week 52 of 2003, nationwide activity exceeded 1.0 case per sentinel, with rapid increase thereafter. In 2004, activity peaked during week 5, then rapidly decreased to less than 1.0 case per sentinel during week 14 (Fig. 1). At the prefectural/district level, recent seasons have demonstrated an early increase in sentinel reporting from Kyushu (see IASR, Vol. 23, No. 12 and Vol. 24, No. 11). However, in 2003/04, cases first began to increase in the Tohoku district (Fig. 2).

Excess mortality due to influenza (see p. 285 of this issue): Excess mortality due to influenza, based on the total number of deaths in Japan, was seen only in February 2004 and estimated at 2,400 cases, the fourth lowest total over the past 10 seasons. Excess mortality due to influenza and pneumonia in 13 large designated cities was posted on the IDSC homepage with a delay of about two weeks from case deaths.

Isolation of influenza viruses: During the 2003/04 season, prefectural and municipal public health institutes (PHIs) nationwide reported a total of 4,740 influenza AH3 virus isolates, 290 influenza B isolates, and only five influenza AH1 isolates (see IASR, Vol. 25, Nos. 2 & 9) (as of October 22, 2004: Table 1).

Influenza AH3 viruses were isolated from four cases in Nagasaki Prefecture during weeks 37-38 of 2003 (see IASR, Vol. 24, No. 11), with subsequent increases throughout the country beginning in week 51 and peaking during week 4 of 2004 (Fig. 1) (http://idsc.nih.go.jp/iasr/prompt/graph/in17.gif). Influenza B virus was isolated in Okinawa during week 36 of 2003 (see IASR, Vol. 24, No. 10), and following isolation in Aichi during week 43, continued to be isolated in small numbers in 39 prefectures from week 47 through week 23 of 2004(http://idsc.nih.go.jp/iasr/prompt/graph/in18.gif). Influenza B isolates from cases of influenza-like illness at an elementary school in Nagoya during week 17 of 2004 (see IASR, Vol. 25, No. 6), and influenza B viruses belonging to the Victoria lineage in Okinawa during February-April, were also reported (see IASR, Vol. 25, No. 9).

During the non-epidemic periods of weeks 25 and 33-36 of 2004, influenza AH3 viruses were isolated from travelers to Asia and Oceania (see p. 290 of this issue), while influenza AH1 viruses were isolated during week 32 from travelers to the Philippines (see IASR, Vol. 25, No. 9).

Influenza AH3 viruses were mainly isolated from cases between 10-14 years of age (27%), followed by 0-4 years (26%) and 5-9 years (20%); the number of isolates from cases in their teens and early 20fs was more than that reported during the 2002/03 season. Seventy-one percent of all influenza B viruses were isolated from cases between 0-9 years of age (Fig. 3).

Antigenic characteristics of 2003/04 isolates and the vaccine strains for 2004/05 season: More than 90% of influenza AH3 viruses isolated during the 2003/04 season were of the A/Fujian/411/2002-like strain, which exhibited less than one-fourth homology to A/Panama/2007/99 (2003/04 vaccine strain) by hemagglutination-inhibition (HI) titers; 9% of isolates were variant strains with low reactivity against any of the reference antisera. Influenza B viruses belonging to the Yamagata lineage were dominant in 2003/04 for the first time in two seasons, with most of these viruses being B/Shanghai/361/2002-like strains (see p. 280 of this issue).

Based on the prevalent influenza viruses circulating in Japan and abroad and the requirements of the vaccine production, A/New Caledonia/20/99(H1N1), A/Wyoming/3/2003(H3N2), a representative of the A/Fujian/411/2002-like virus, and B/Shanghai/361/2002, belonging to Yamagata lineage, have been chosen as the vaccine strains for the 2004/05 season (see IASR Vol. 25, No. 9).

Production of influenza vaccine and influenza vaccine coverage rate among the elderly: Approximately 20 million vials of influenza vaccine have been produced for the 2004/05 season, one million of which have been designated as reserve supply in the event of a vaccine shortage (see p. 288 of this issue). Vaccine coverage rates among the elderly (primarily those over 65 years of age), in compliance with the Preventive Vaccination Law, were 27% for the 2001/02 season, 35% for the 2002/03 season, and 45% for the 2003/04 season (Blood and Blood Products Division, Pharmaceutical and Food Safety Bureau, Ministry of Health, Labour and Welfare).

Avian influenza A(H5N1) outbreaks: In February 2003, human cases of avian influenza A(H5N1), one of them fatal, were reported in Hong Kong (see WER, Vol. 78, No. 10, 2003). In association with outbreaks of influenza A(H5N1) virus infection among birds in Asia in 2004, 17 cases (12 deaths) in Thailand and 27 cases (20 deaths) in Vietnam of human infection were reported to WHO (total between October 2003-October 25, 2004) (see p. 293 of this issue and go to http://www.wpro.who.int/avian/).

Influenza A(H5N1) outbreaks also occurred among birds in Japan: Yamaguchi (a poultry farm) (see p. 294 of this issue), Oita (pet bantams), Kyoto (two poultry farms) (see p. 295 of this issue), and Hyogo (a poultry farm). Approximately 275,000 domestic fowl either died of infection or were slaughtered. However, no cases of human infection were identified. Due to thorough countermeasures to contain the spread of disease, such as culling of chickens on contaminated farms, limited entry into contaminated farms and their surroundings, restricted movement of goods and animals, and disinfection (see p. 297 of this issue), no additional outbreaks have occurred since the declaration of cessation in April 2004.

Preparedness for the next influenza pandemic: In Japan, a working group (lead by Y. Hirota) has been organized to begin preparations for the next influenza pandemic. The groupfs reports were summarized in August 2004 (see p. 287 of this issue and http://www.mhlw.go.jp/topics/2004/09/tp0903-1.html).

Preliminary reports for 2004/05 season: Influenza AH3 virus was isolated from a case in Aichi (week 36) (see p. 290 of this issue), from one case (week 39), four cases (week 41), and one case (week 43) in Osaka (see p. 290&291 of this issue), from two cases in Tokyo (weeks 42-43), and from three additional cases by PCR in Tokyo (as of November 2, 2004) (refer to http://idsc.nih.go.jp/iasr/prompt/graph-k.html).

During week 42, a case of influenza encephalopathy in a 6-year-old boy was reported as gacute encephalitish from Tokyo. Antibody prevalence prior to the 2004/05 season, under the National Epidemiological Surveillance of Vaccine-Preventable Diseases, will be posted on the IDSC homepage as soon as data become available.

On October 25, designated as a kickoff day, efforts have begun toward developing an integrated strategy against influenza for this upcoming winter (see p.287 of this issue).


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