Pandemic (H1N1) 2009 Transmission during Presymptomatic Phase, Japan Yoshiaki Gu, Nobuhiro Komiya, Hajime Kamiya, Yoshinori Yasui, Kiyosu Taniguchi, and Nobuhiko Okabe During an epidemiologic investigation of pandemic influenza (H1N1) 2009 virus infection in May 2009 in Osaka, Japan, we found 3 clusters in which virus transmission occurred during the presymptomatic phase. This finding has public health implications because it indicates that viral transmission in communities cannot be prevented solely by isolating symptomatic case-patients. T he first indigenous cases of pandemic (H1N1) 2009 in Japan were detected in Kobe City (1) and Osaka Prefecture on May 16, 2009. In response to the outbreak, the National Institute of Infectious Diseases, Infectious Diseases Surveillance Center, and its Field Epidemiology Training Program began epidemiologic investigations in both areas. Clinical manifestations of these infections were described in 2 previous articles (2,3). In general, pandemic influenza (H1N1) 2009 virus is considered to be infectious during patients’ presymptomatic phase (4,5). However, to our knowledge, no epidemiologic studies about infectiousness during the presymptomatic phase have been reported. The aim of this study was to provide scientific evidence to ascertain the infectious period of pandemic influenza (H1N1) 2009 through epidemiologic investigation in Osaka, Japan. The Study We began an epidemiologic investigation in Osaka on May 17, 2009, and conducted face-to-face interviews. Thirty-six confirmed cases had occurred by May 22. The definition of a confirmed case-patient was a person with influenza-like illness (ILI) and laboratory confirmation of pandemic (H1N1) 2009 virus infection by real-time reverse transcription PCR. ILI was defined as the presence of a fever (>38.0°C) and acute respiratory symptoms (cough or sore throat). Local public health staff also conducted face-to-face or telephone interviews, or both, with other patients and persons in close contact with the case-patients until the end of May, when the local epidemic appeared to wane (1,6). Interviews were conducted by using a standard questionnaire to collect data on patient demographics, clinical course of illness, patient behavior, and history of patient contacts. By analyzing these data and careful investigation of contact history and epidemiologic links, we made a transmission tree. During the epidemiologic investigation, we found 3 clusters in which disease transmission could have occurred before symptom onset in the index case-patients (Figure). The first outbreak in Osaka occurred in 1 high school, and no community transmission had been verified in mid- May. Case-patient 1, a student at the high school, had symptoms on May 13. Case-patient 2, the sister of case- patient 1, had no symptoms while she was at school, according to her answer on the self-report survey as well as face-to-face interview carried out with a classmate who sat right behind her (case-patient 3). Symptoms developed in the evening of May 15 after she returned home, and the school was closed proactively the following day. She later received a diagnosis of infection with the virus. No other students or staff had ILI symptoms at that time. For case-patient 3, who sat immediately behind case-patient 2 in class and chatted with her, influenza-related symptoms developed on May 17. Our investigation suggested that transmission of infection from case-patient 2 to case-patient 3 occurred during the presymptomatic phase of infection in case-patient 2. Case-patients 4, 5, and 6 were previously healthy boys and schoolmates of case-patients 2 and 3. They did not have contact with anyone with ILI symptoms at school or Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 9, September 2011 1737 Author affiliation: National Institute of Infectious Diseases, Tokyo, Japan DOI: http://dx.doi.org/10.3201/eid1709.101411 Figure. Three clusters of pandemic (H1N1) 2009 presymptomatic transmission in May 2009 in Osaka, Japan. All cases were confirmed as pandemic (H1N1) 2009 virus infection by real-time reverse transcription PCR. Squares indicate male case-patients, and circles indicate female case-patients. Colors of the squares and circles denote the similar or different schools the students attended.