Abstract
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 Rt moved below 1 by 17 May. Active surveillance and public health interventions, including school closures most likely have contributed to keeping Rt below one.
Highlights
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]
The exclusion of the less documented cases in Kobe lead to an R estimate of 2.0
The pandemic potential of this virus in Japan may be higher in terms of transmission potential than in other areas of the world
Summary
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]. The present study investigates indigenous secondary transmissions of the new influenza A(H1N1) virus in Japan, estimating R and exploring its age-specificity. Cases outside Osaka and Hyogo prefectures had travel histories to Osaka or Hyogo before their illness onset. The index case(s) (who may have remained asymptomatic [7]), with a history of overseas travel, has (have) yet to be identified. The triggering event may be associated with Japan’s two-week festive break, the “golden week”, just before 9 May, when people may have travelled to and returned from Mexico, United States and Canada
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