Avian Influenza A (H7N9) Virus

Human infections with a new avian influenza A (H7N9) virus were first reported in China in March 2013. Most of these infections are believed to result from exposure to infected poultry or contaminated environments, as H7N9 viruses have also been found in poultry in China. While some mild illnesses in human H7N9 cases have been seen, most patients have had severe respiratory illness, with about one-third resulting in death. Rare, limited person-to-person spread of this virus has been identified in China, but there is no evidence of sustained person-to-person spread of H7N9. The first case outside of China was in Malaysia and was reported on February 12, 2014. The case was detected in a traveler from an H7N9-affected area of China. The new H7N9 virus has not been detected in people or birds in the United States.

It’s likely that sporadic cases of H7N9 associated with poultry exposure will continue to occur in China. It's also possible that H7N9 may spread to poultry in neighboring countries and that human cases associated with poultry exposure may be detected in those neighboring countries. It’s also possible that H7N9 cases may continue to be detected among travelers returning from H7N9-affected countries, even possibly in the United States at some point. However, as long as there is no evidence of ongoing, sustained person-to-person spread of H7N9, the public health risk assessment would not change substantially. Most concerning about this situation is the pandemic potential of this virus. Influenza viruses constantly change and it’s possible that this virus could gain the ability to spread easily and sustainably among people, triggering a global outbreak of disease (pandemic). The U.S. Government supports international surveillance for H7N9 and other influenza viruses with pandemic potential. CDC is following the H7N9 situation closely and coordinating with domestic and international partners. Most important, is CDC takes routine preparedness actions whenever a new virus with pandemic potential is identified, including developing a candidate vaccine virus to make a vaccine in case vaccine is needed. Those preparedness measures continue. CDC also has issued guidance to clinicians and public health authorities in the United States, as well as provided information for people traveling to China. CDC will provide updated information as it becomes available.

Background

During the spring of 2013, the World Health Organization (WHO) reported 132 human H7N9 infections, with 44 deaths. Most cases had illness onset during the month of April. Beginning in May, new reports of human H7N9 infection in China became less frequent. From June to the end of September 2013, WHO reported three new H7N9 infections in China; one had illness onset in April, and one resulted in death. The decrease in H7N9 cases over the summer likely resulted from a combination of control measures taken by Chinese authorities - like closing live bird markets - and the change in weather. Studies indicate that avian influenza viruses, like seasonal influenza viruses, have a seasonal pattern: they circulate at higher levels in cold weather and at lower levels in warm weather.

In the beginning of October, the frequency of reports of human infection with H7N9 began to increase. WHO and China reported more new H7N9 cases in China per month relative to the summer months, including three cases reported by Hong Kong Special Administrative Region of People’s Republic of China in early December. These cases coincided with the arrival of cooler weather in China and were not unexpected. Most of the cases that were reported had poultry exposure and lived in areas where H7N9 had been found previously. As of mid-January, cases continue to be reported and the frequency of these reports has increased. Although epidemiological investigations are ongoing for some of the more recent cases, currently no evidence has been found that indicates sustained human-to-human transmission is occurring.