From Centers for Disease Control and Prevention Web site-
A new bird flu virus, called H7N9 for short, is infecting people in China and causing serious illness. About 20 percent of the people who have been infected have died. Luckily, H7N9 is not spreading easily between people the way seasonal influenza (flu) viruses do. Available evidence indicates that most people are being infected after exposure to birds or to environments that might be contaminated with bird flu virus (like live bird markets, for example). There have been no cases of H7N9 in the United States or anywhere outside of China.
Some people might ask why CDC is concerned about a virus that is causing illness on the other side of the world. Should people in the United States be worried?
Risk in the United States
At this time, the risk of getting sick from H7N9 in the United States is low. CDC does not have any new or special recommendations for the U.S. public associated with H7N9, known scientifically as avian influenza A (H7N9). But the emergence of this new H7N9 bird flu virus as a cause of severe illness and death in people raises some serious public health concerns everywhere.
Wild water and shore birds are hosts to many avian influenza viruses. These viruses are very different from human influenza viruses. Mostly, they don't spread easily from birds to people, so human infections with bird flu are quite rare. But because these viruses are so different from human flu viruses, most people have little to no immunity. For that reason, whenever there is a human infection with a new animal virus (a "novel" virus), scientists sit up and take notice. With the H7N9 situation in China, several things are worrisome:
The H7N9 virus has genetic changes that have been associated with making bird flu viruses spread better and more easily to mammals, causing more severe disease and causing disease to progress more quickly.
Human H7N9 virus infections so far have resulted mostly in very serious illness. Cases have started with flu-like symptoms including fever and cough and progressed quickly to severe pneumonia, acute respiratory distress syndrome (ARDS) and multi-organ failure.
In the two months after H7N9 was first detected, more than 120 human cases were reported, including 24 deaths.
Spread of H7N9
It's reassuring that no evidence of ongoing human-to-human spread of this disease has been found. Chinese health officials have carefully followed up on more than 2,000 close contacts of confirmed cases and have not found any indication of community spread. Also, China's regular surveillance system for influenza-like illness has not shown any unusual increase in activity.
But influenza viruses are constantly changing, and a big concern is that this H7N9 virus might gain the ability to spread easily from person to person. This can happen when animal and flu viruses mix genetic information (reassort) and produce a new influenza virus that can spread easily between people but that is still very different from human viruses, and so people would have little or no immunity against it. Such "reassortment" events are believed to have happened before the influenza pandemics of 1918, 1957, 1968 and 2009. CDC doesn't know if this change will occur. But if this change happens with H7N9, the severity of illness associated with this virus is concerning. The world could face a pandemic with a high toll in illnesses, hospitalizations and deaths.
What is CDC doing about H7N9?
CDC is following this situation closely and coordinating with domestic and international partners, including the Chinese Center for Disease Control (China CDC) and the World Health Organization (WHO), and taking aggressive proactive steps to be ready for the possibility that this virus may change to spread easily between people.
On April 11, 2013, the Influenza Division laboratory at CDC received one of the H7N9 viruses from China.
CDC has been using the virus for the following routine preparedness measures:
-Develop a test kit for detecting H7N9 infections in humans that is being shared with approved public health laboratories. The kits have already been distributed to most U.S. states and are being shared internationally, too.
-Test for the presence of antibodies against the H7N9 virus in human blood samples. This will allow CDC to see if some people already have immunity against this virus.
-Test to see if certain existing antiviral drugs (i.e. oseltamivir [Tamiflu®] and zanamivir [Relenza®]) will work to treat H7N9. (Visit H7N9 Update; CDC Pandemic Preparedness Activities Progress for more information.)
-Develop a candidate vaccine virus that could be used to make a vaccine if it is needed
Taken from Centers for Disease Control and Prevention Web site. https://www.cdc.gov/Features/h7n9flu/index.html