European Centre for Disease Prevention and Control

Origins of the new influenza A(H1N1) virus: time to take action 12 On 16 May 2009, Japan confirmed its first three cases of new influenza A(H1N1) virus infection without a history of overseas travel, and by 1 June, 361 cases, owing to indigenous secondary transmission, have been confirmed. Of these, 287 cases (79.5%) were teenagers (i.e. between 10 and 19 years of age). The reproduction number is estimated at 2.3 (95% confidence interval: 2.0, 2.6). The average number of secondary transmissions involving minors (those under 20 years of age) traced back to infected minors is estimated at 2.8. That is, minors can sustain transmission even in the absence of adults. Estimates of the effective reproduction number R t moved below 1 by 17 May. Active surveillance and public health interventions, including school closures most likely have contributed to keeping R t below one. Introduction The reproduction number R, the average number of secondary cases generated by a single primary case, of the new influenza A(H1N1) virus, is a key quantitative measure for assessing pandemic potential [1]. In the ongoing epidemic of the new influenza A(H1N1) virus, early studies suggested that R ranged from 1.4-1.6 [2] and some estimated it to be as high as 2.2-3.1 [3]. Estimates in 1.4-1.6 range for the new influenza A(H1N1) virus are lower than estimates based on data from, for example, the fall wave of the 1918 influenza pandemic [4,5]. The present study investigates indigenous secondary transmissions of the new influenza A(H1N1) virus in Japan, not only estimating R but also exploring its age-specificity. Methods Epidemiological description of the epidemic On 16 May 2009, three high school students in Kobe city, Hyogo prefecture, without a history of overseas travel, were confirmed as infected with the new influenza A(H1N1) virus. Confirmatory diagnosis in Japan requires influenza-like symptoms and a laboratory diagnosis which is made either by virus isolation, real-time PCR or a significant increase in neutralising antibody titre against the virus. Further confirmed diagnoses followed predominantly in Hyogo and Osaka prefectures. The increased number of infections among particular age groups was most evident in the data from prefectures where most secondary cases were found among high school students attending different schools. By 1 June, the Ministry of Health, Labour and Welfare of Japan had reported 371 confirmed cases, including nine imported cases and one case traced back to a distant international airport (i.e. a worker at …