How has the H7N9 virus situation developed and changed in China since 2013?
In December 2016, U.S. CDC and the Chinese CDC published a collaborative study regarding the H7N9 virus outbreaks in China during the first four annual epidemics. The study assessed what public health experts have learned about the H7N9 virus and how the situation has changed since human infections were first identified in early 2013. Looking at the annual epidemics from an epidemiological standpoint, H7N9 virus infections of humans have expanded to affect a larger geographic area of China. During the 4th epidemic, more cases of H7N9 virus infection were reported in people living in rural areas. Also during the 4th epidemic, public health researchers observed an increase in the proportion of people who became severely ill. People with H7N9 virus infection were significantly more likely to develop pneumonia and be admitted to an intensive care unit (although they were not more likely to die) than in past epidemics. Also, the 4th epidemic lasted longer than the previous three epidemics.
However, some aspects of the H7N9 situation in China have remained consistent to date. There has been no evidence of increased person-to-person transmission of H7N9 virus. In addition, the age and gender distribution of patients and the history of exposure (primarily recent exposure to live poultry, including by visiting a live poultry market) have remained consistent. Analysis of the genetic properties of H7N9 viruses looking for adaptations that would result in antiviral resistance or adaptation to more easily infect mammals indicates that recent H7N9 viruses are similar to what has been observed for H7N9 viruses throughout the four previous epidemics. Researchers are continuing to monitor the genetic properties of H7N9 viruses during the current 5th epidemic to look for changes.
The H7N9 virus continues to have the greatest potential to cause a pandemic of known emerging influenza A viruses, and H7N9 viruses are considered to be the influenza A virus with the greatest potential public health impact (i.e., the potential severity of human disease caused by the virus), according to CDC's Influenza Risk Assessment Tool (IRAT). As a result, CDC, China and global health partners will continue to closely monitor the H7N9 virus situation in China and will continue to conduct risk assessments as the situation evolves.