Infectious Disease Sureillance Center

IDWRNovel Influenza A(H1N1) (8 June 2009)

Interim report on two clusters of the novel influenza A (H1N1) infection in Osaka Prefecture

May 21, 2009

Infectious Disease Surveillance Center, National Institute of Infectious Diseases
Osaka Prefecture and Pubulic Health Center of Osaka Prefecture


The majority of novel influenza A (H1N1) virus infections reported from the Osaka area were limited to schools. As of 20th May, 99 confirmed cases have been reported, of which 64 were from a single private integrated junior and senior high school. From 18th May, the prefecture changed the previous policy of compulsory hospitalization to the treatment at home for most of patients based on clinical indications. Here we report clinical information on 69 cases from two clusters (the previously mentioned private school and a nearby elementary school).

Cluster 1

  • setting: integrating junior and senior high school, with 1,934 students and 143 teachers
  • study population is 64 confirmed cases ( 59 high school students, 2 junior highs students, 3 teachers), male 49, female 15
  • median age: 16 y.o. (range: 13 to 53)
  • no oversea travel history reported after April 2009
  • 18 hospitalized and 46 medical observation at home
  • commute from wide areas ( e.g. Northern Osaka to western Kobe)
  • limited school bus service available

<Course> From 11th May, increased reporting of fever was observed, mainly in the sophomore class of high school. Twenty absentees were reported among sophomores on 12th May which increased to 36 on the next day. Temporary whole grade closures were in place from 13th to 15th May, following seasonal influenza procedures. On 16th May, samples from influenza A positive students were sent for RT-PCR testing to the public health laboratory and then to National Institute of Infectious Diseases.

<Data collection> Direct face-to-face interviews were carried out on 18 hospitalized patients, and telephone interviews were performed on 46 home-quarantined patients by school teachers with our technical assistance.

<Symptoms and treatment> Almost all of the cases had clinical features compatible with seasonal influenza. No cases were reported with serious conditions. In some cases, respiratory symptoms and/or vomiting preceded high fever by a few days.

Antiviral medications were given to all hospitalized cases: oseltamivir to 12 patients and zanamivir to the remaining 6. All cases recovered relatively quickly and uneventfully.


  • elementary school: 624 students
  • study population: 5 confirmed cases , male: 2 , female: 3
  • age distribution (one 9 y.o., three 11 y.o., one 12 y.o)
  • no underlying conditions noted

 <Course> On 16th May, a sixth grade female student visited the weekend care clinic and was diagnosed as influenza A positive by a rapid kit test. Zanamivir was prescribed and her sample was sent to the regional public health laboratory for further diagnosis. She was confirmed as a case on the 17th and cases increased to 5 until the 19th.

<Symptoms and treatment> Clusters reported from elementary schools are still rare in Japan. No case has been reported in serious condition and all have recovered relatively quickly and uneventfully after diagnosis. In two cases, respiratory symptoms preceded high fever by a few days.

Rapid test results

The sensitivity of the rapid test changed by the Day examined; it peaked on the Day 1 and was relatively low on Days 0 and 2. It is not possible to determine the effectiveness of the rapid test kit from the data presented here because of insufficient information (e.g. type of kit used). However, it may be premature to exclude novel influenza A (H1N1) only from the result of rapid kit test.

Table3.Rapid kit test results of RT-PCR positive confirmed cases of novel influenza A (N1H1)(N=23)

We express deep appreciation for the cooperation and support of the members and staffs of Osaka City General Hospital, Toyonaka Municipal Hospital, Sakai Municipal Hospital, Osaka City Public Health Centre, Sakai City Public Health Centre, and Takatsuki City Public Health Centre.

(2009/6/8 IDSC)

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