On April 1, 2013, the World Health Organization (WHO) first reported 3 human infections with a new influenza A (H7N9) virus in China. Since then, additional cases have been reported. Most reported cases have severe respiratory illness and, in some cases, have died. At this time, no cases of H7N9 outside of China have been reported. The new H7N9 virus has not been detected in people or birds in the United States.
This new H7N9 virus is an avian (bird) influenza (flu) virus. Human infections with avian influenza (AI, or “bird flu”) are rare but have occurred in the past, most commonly after exposure to infected poultry. However, this is the first time that this bird flu subtype (H7N9) has been found in people. This virus is very different from other H7N9 viruses previously found in birds.
An investigation by Chinese authorities is ongoing. H7N9 viruses have been detected in poultry in the same area where human infections have occurred. Many of the human cases of H7N9 are reported to have had contact with poultry. However some cases reportedly have not had such contact. Close contacts of confirmed H7N9 patients are being followed to see if any human-to-human spread of H7N9 might have occurred. Based on previous experience with other avian influenza viruses – most notably H5N1 – some limited human-to-human spread of this H7N9 virus would not be surprising. Most importantly, however, no sustained person-to-person spread of the H7N9 virus has been found at this time. Ongoing (sustained) person-to-person spread is necessary for a pandemic to occur.
This is a “novel” (non-human) virus and therefore has the potential to cause a pandemic if it were to change to become easily and sustainably spread from person-to-person. So far, this virus has not been determined to have that capability. However, influenza viruses constantly change and it's possible that this virus could gain that ability. CDC takes routine preparedness actions whenever a new virus with pandemic potential is identified, including developing a candidate vaccine virus to make a vaccine if it were to be needed. There is no licensed H7 vaccine available at this time.
CDC is following this situation closely and coordinating with domestic and international partners. In addition, CDC has issued guidance to U.S. clinicians and public health departments on how to test for this virus, and interim guidance on case definitions for possible H7N9 cases in the United States and interim infection control guidance for U.S. health care workers. CDC also has developed information for travelers to China.
The sequences of the first three viruses were posted to GISAID by China and are publicly available. Some of the genetic changes have been associated with increased transmissibility of other AI viruses to mammals based on animal studies involving ferrets in the past. So far, all three viruses seem to be susceptible to the influenza antiviral drugs oseltamivir and zanamivir, but they are resistant to the adamantanes.
This is an evolving situation and there is still much to learn. CDC will provide updated information as it becomes available.