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While people in China watch the slow spread of the new bird flu, scientists worldwide are trying to figure out if it could present a bigger threat. Researchers say they already know a lot about the new flu, but NPR's Richard Knox reports that even the experts have to wait for the virus to declare if it's going to cause the next big flu pandemic.

RICHARD KNOX, BYLINE: There's been a buzz of activity at the Centers for Disease Control and Prevention in Atlanta since scientists got their first samples of the new bird flu virus from China four weeks ago.

DR. DANIEL JERNIGAN: At this point, we're following this very closely because this is a virus that is new, we have not seen it before.

KNOX: That's Dr. Daniel Jernigan, one of the CDC's top flu experts. There are two reasons why the virus called H7N9 is getting so much attention. First, people who get it usually get very sick, and so far nearly a quarter of them have died. Second, the genetic makeup of this new virus is disturbingly different from an older bird flu that has sickened more than 600 people over the past 10 years and killed more than half of them. That virus is an H5 type.

JERNIGAN: The thing that's different between them is the H5 virus still maintains a lot of the avian or bird flu characteristics, whereas this H7N9 shows some adaptation to mammals. And that's what makes it different and concerning for us.

KNOX: For instance, the newer bird flu has adapted to thrive at human body temperature, which is much lower than birds. It's a necessary step toward becoming a flu that could spread easily from human to human, but it's not enough.

It still has a ways to go before it becomes like a human virus. But the fact is, it's somewhere in that middle ground between purely avian and purely human.

Dr. Anthony Fauci of the National Institute of Allergy and Infectious Diseases says that's a red flag.

DR. ANTHONY FAUCI: That's something you have to keep an eye on. But we still don't know the complete road map of how it goes from a relatively non-adaptable to an adaptable virus to humans.

KNOX: Meanwhile, officials aren't just waiting to see if this virus becomes fully adapted. The CDC has asked state and local officials to dust off the plans they worked up several years ago when it looked like the earlier bird flu virus might cause a big pandemic. Fauci says serious planning sessions are under way throughout the federal government.

FAUCI: The pandemic preparedness engines are already humming.

KNOX: For instance, government scientists have provided samples of the H7N9 virus to drug companies to make into batches of experimental vaccine.

FAUCI: We would hope that there's enough vaccine by the end of June or so, so that you could start the clinical trials through July, August, towards the end of the summer. Hopefully it won't be any later than then.

KNOX: Those studies will tell scientists if they have a good enough vaccine against the new flu, in case the virus begins to spread easily among people. Meanwhile, the problem is how to prevent people from getting it from chickens. In the past, that's been accomplished by wholesale killing of poultry flocks as soon as any get infected. This week that's what Chinese officials did at a chicken farm in Guangdong, the southern province that provides Hong Kong with most of its poultry; 90,000 chickens were destroyed after the H7N9 virus was found in one bird. But Jernigan of the CDC says that won't work as a general strategy because this new virus doesn't make chickens sick, even though they can be infected and spread the virus around.

JERNIGAN: For this particular virus, which we know can cause severe disease in humans but does not necessarily cause severe disease in the poultry, it makes it very difficult to know where that virus is.

KNOX: After all, health officials can't just kill every chicken in China.

JERNIGAN: It is a difficult problem. There are lots of chickens in China.

KNOX: And even if a chicken vaccine were developed, it might be hard to get farmers to use it since this virus doesn't kill chickens. Richard Knox, NPR News.

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