Pandemic influenza A (H1N1) situation report of Japan, update 21
November 6, 2009


According to the National Epidemiological Surveillance for Infectious Diseases in Japan, the numbers of reported cases of the Influenza like Illness (ILI) increased in the Epidemiological Week 42 (October 12-18, 2009) (Figure 1). The number of cases reported per sentinel* was 17.65, increased from 12.92 in previous week. Newly diagnosed ILI patients during this week are estimated to be approximately 0.83 million nationwide based on this number.

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

Hokkaido reported the highest cases per sentinel (57.93), followed by Aichi Prefecture (31.78) and Fukuoka Prefecture (29.08). Hokkaido has 30 community health centers (HCs) of which 23 health centers (77%) were above the alert level (30.00) and 4 HCs (13%) were above warning threshold (10.00). For the details, refer to the Map of ILI epidemic level at following URL (Japanese only):

According to the ILI report from Ministry of Health Labor and Welfare (MoH), in week 42 (Figure 2), 619 schools in Japan, have been closed due to the ILI outbreak within the schools. In addition, one whole grade was suspended in 2,051 schools and a class was closed in 5,864 schools with multiple infected students.



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


There were 445 reported hospitalization cases due to pandemic influenza A (H1N1) infection during October 14 to 20, according to MoH.

According to the Infectious Agents Surveillance Report, over 99% of the influenza viruses detected/isolated from specimens collected from influenza cases during weeks 19 to 43 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.