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H7N9 Update

October 28, 2013 -- Rare, sporadic human infections with H7N9 in China have been reported since the end of May, following a surge of more than 130 cases with illness onset during the month of April. Most recently, China reported to the World Health Organization that two cases of H7N9 infection were detected there in October. Information about these cases was posted on the WHO website on October 16 and October 24. These are the first cases of H7N9 reported since August 11 and bring the total number of cases reported to 137, including 45 deaths. These most recent cases are not unexpected and do not change the risk assessment for H7N9 at this time.

As CDC indicated in May 2013, evidence suggests that there is a seasonal pattern to the circulation of avian influenza viruses, with activity declining in warmer seasons and increasing in cooler weather. This suggests that H7N9 activity – among birds and people – might increase again in China as the weather turns cooler. It’s likely that H7N9 cases will continue to be detected this winter in China, however, it’s not possible to predict how many cases there will be.

The most recent case of H7N9 in China reportedly occurred in a farmer who had contact with live poultry, suggesting that exposure to infected birds continues to be the main source of infection with this virus. There is no indication that the epidemiology of H7N9 has changed; China continues to report that there is no evidence of sustained human-to-human transmission with this virus. If more clusters of human-to-human spread, or evidence of sustained human-to-human spread were detected, the risk assessment for H7N9 would change. In the meantime, H7N9 preparedness efforts by the U.S. government have continued over the summer and CDC along with its global health partners are continuing to watch this situation closely.

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