Introduction and transmission of 2009 pandemic influenza A (H1N1) Virus--Kenya, June-July 2009.

In April 2009, in the United States, the first cases of 2009 pandemic influenza A (H1N1) virus infection were reported. On June 11, the World Health Organization (WHO) declared an influenza pandemic because of widespread transmission worldwide. As of September 13, all six WHO regions had reported approximately 296,471 cases of pandemic H1N1, including 3,486 deaths. On June 29, the first case of pandemic H1N1 was confirmed in Kenya. At that time, only four other countries in sub-Saharan Africa had reported cases, and secondary transmission had not been documented in the region. Surveillance activities in Kenya detected four separate introductions of the virus into the country. The introductions provided an opportunity to study transmission of the virus (including calculation of secondary household attack rates) in a virus-naive population that had not yet initiated the use of antiviral drugs. This report describes the four introductions and the accompanying analysis. The overall rate of secondary household transmission of laboratory-confirmed pandemic H1N1 was 26% (range: 7%-33%), which is comparable to secondary household attack rates reported for laboratory-confirmed seasonal influenza virus infection. However, additional and more rigorous studies are needed to better understand the secondary attack rates associated with the current pandemic.