Epidemiological investigation of a novel influenza A (H1N1) outbreak detected by entry screening, Narita, Japan, 2009 -- Preliminary report

May 19, 2009

Infections Disease Surveillance Center and Field Epidemiology Training Program, National Institute of Infectious Diseases

Updated on May 21, 2009


On April 24 2009, a group of 30 high school students (14 second year students and 16 third year students) and 6 teachers from 3 schools in Osaka travelled to Toronto via Detroit on an international exchange program run by the city governments. Among several activities during their stay, the participants did homestays with local families and sat in on some classes with local high school students.  On May 8, they came back to Narita airport via Detroit airport. Three members of the group presenting with symptoms were confirmed to have novel influenza A (H1N1) infection through entry screening and were hospitalized in the designated hospital for isolation. The other 33 participants, as well as 14 passengers who had seats near the confirmed cases and 2 airflight crew members were instructed to stay under medical supervision at the designated accommodations. Among them only one participant developed symptoms and was confirmed to be infected with novel influenza A (H1N1) during the period of medical supervision.

As per a request from the Ministry of Health, Labour and Welfare (MHLW), a team from the Infectious Disease Surveillance Center and Field Epidemiology Training Program at the National Institute of Infectious Diseases (NIID) was dispatched and performed an epidemiological investigation of the participants of the international exchange program. Suspected cases were defined as those participants of the exchange program that had high fever (>=38℃) OR at least two acute respiratory symptoms* with onset between April 24 and May 15 (Canada time). Among suspected cases, confirmed cases were defined as those persons who were positive for novel influenza A(H1N1) virus infection that was laboratory confirmed by real-time PCR at Narita Quarantine laboratory and NIID. These case definitions were based on MHLW case definitions.

Four confirmed cases (three students and one teacher) and eight suspected cases (all students) were detected. One school had four confirmed cases and three suspected cases and the other two schools had three and two suspected cases, respectively. The date of onset was from May 5 to 8 for confirmed cases, and from April 25 to May 12 for suspected cases. Among confirmed cases, four had high fever (>=38℃) and cough, of which three had a sore throat and joint pain and either nasal obstruction or rhinorrhea. One confirmed cases had abdominal pain but none of the four confirmed cases had vomiting or diarrhea. Some suspected cases tested negative for novel influenza A(H1N1) infection by real-time PCR, but no further details were obtained. None of the suspected cases had high fever (>=38℃).

One of the three high schools sent 10 students (6 female, 4 male) and 2 teachers (both male) on the international exchange program.  Of these, there were four confirmed cases. Confirmed student cases were all male.  These three students spent a lot of time together during the exchange program.  In addition, one teacher (also a confirmed case) accompanied one of the three students to see a physician in Canada. The date of onset for suspected cases at this school ranged between May 2 to May 8. The date of onset for the four confirmed cases fell within this period (May 5 to 8). The likely source of infection was not discovered through interviews of the cases and other participants on the trip.

In the afternoon of May 18, all persons under medical supervision were released with no additional cases discovered.  The 4 confirmed cases have all recovered and are considered to pose no risk of further transmission.  Additionally, negative results were obtained for all cases by RT-PCR.  By May 20, all four had been discharged from the hospital.


* Acute respiratory symptoms include: