Infectious Disease Sureillance Center

IDWRPandemic (H1N1) 2009

Pandemic influenza A (H1N1) situation report of Japan, update 25
December 11, 2009

According to the National Epidemiological Surveillance for Infectious Diseases in Japan, the numbers of reported cases of the Influenza like Illness (ILI) have been decreased in the 49th epidemiologic week (November 30-December 6, 2009) (Figure 1). The number of cases reported per sentinel* in Japan was 31.82, which has decreased compared to previous week (39.63/sentinel). Based on this number, newly diagnosed ILI patients during this week are estimated to be approximately 1.5 million nationwide.

Figure 1.  Influenza (like illness) cases reported weekly per sentinel

During the 49th epidemic week (November 30-December 6, 2009), influenza activity has decreased all Prefectures, except for two prefectures (Aomori and Tokushima). In comparison with the last week, Fukui Prefecture decreased 36% (from 95.44/sentinel to 60.66/sentinel) which was the highest rate of reduction, followed by Fukuoka Prefecture (34%; from 63.35/sentinel to 41.99/sentinel) and Aichi Prefecture (33%; form 49.95/sentinel to 33.37/sentinel). On the other hand, thirty two Prefectures (68%) still in alert level (30.00/sentinel or over). Fukui Prefecture had the highest reported cases per sentinel (60.66), followed by Oita Prefecture (54.69), and Yamaguchi Prefecture (54.46). For the details, refer to the Map of ILI epidemic level at following URL (Japanese only)

In week 49, according to the School absenteeism surveillance report from Ministry of Health Labor and Welfare (MHLW), 777 schools throughout the nation have been suspended their all classes due to the influenza A (H1N1) 2009 epidemic in school. In addition, one or several grade was suspended in 3,280 schools and 8,905 schools have suspended one or several individual class with multiple infected students.

Figure 2. The number of closed class, grade or school and ILI surveillance by week

There were 656 reported hospitalization cases due to pandemic influenza AH1 pdm infection during December 2 to 8, according to MHLW. By age group, the largest report (264 cases; 40% of all age) remained in the 5-9 year-olds, followed by the 1-4 year-olds (169; 26%) and the 10-14 year-olds (72; 11%). Acute encephalopathy was diagnosed in 22 cases (3%) and mechanical ventilator was used in 31 (5%) among all inpatients.

As of December 8, 2009, cumulative number of the hospitalized cases was 11,942 (Cumulative number of the hospitalized case included case who has already hospitalized on July 28 and new inpatient case on July 29 or later.). Among the hospitalized case, 4,206 cases have some kind(s) of underlying disease, in which 2,751 cases (65%) have had chronic respiratory illness. Cumulative number of the death was 107. Of all death cases, 74 cases (69%) have underlying disease and 10 cases (9%) have encephalitis, based on the infectious disease control law.

According to Infectious Agents Surveillance Report, most of the influenza viruses detected/isolated from specimens of influenza cases during the week 19 through 49 were AH1pdm (Figure 3).

Figure 3. Weekly reports of influenza virus isolation/detection

* The number of sentinel clinics and hospitals is decided from the calculation based on relative population of the jurisdiction of each public health center and the adjustments with consideration enabling comprehend the incidents in the whole area of the prefecture. The sentinel points for influenza like illness consist from approximately 3,000 pediatric hospitals/clinics and 1,800 internal medicine hospitals/clinics around the country.

(2009/12/25 IDSC)

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