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(See the Major Article by Liu et al on pages 787–94.)

As of 20 May 2014, 439 confirmed cases of human A(H7N9) virus infection were reported to the World Health Organization (WHO) [1]. All cases, including those that were detected in Chinese Taipei, Hong Kong Special Administrative Region, and Kuala Lumpur, Malaysia, were presumably infected in mainland China. The cases have occurred in 2 waves: an initial wave (n = 133) during February–May 2013 and a second wave starting in October 2013. This virus has persisted in poultry with a seasonal pattern similar to that of other influenza viruses, circulating at higher levels in cold weather and at lower levels in warmer seasons.

Although complete epidemiologic and virologic analysis of the second wave is yet to be published, risk assessment–related information shared between China and WHO appears to indicate that the host and virus characteristics have not changed substantially between the 2 waves [2]. Most of these infections are believed to result from exposure to infected poultry or contaminated environments. Although some mild illnesses in human A(H7N9) cases have been detected, particularly among younger individuals, most patients have had severe respiratory illness, with about one-third of cases resulting in death. There were no significant increases of clusters during the second wave, and clusters remain limited in size. Epidemiological patterns for A(H7N9) cases from the first wave likely remain valid [3]. Because influenza viruses constantly change, the pandemic potential of this virus persists. As the extent of transmission in poultry is unknown, a surge of sporadic cases of A(H7N9) associated with poultry exposure can be anticipated next winter in eastern China.

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