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In an exclusive interview at the Reuters Health Summit in New York, Dr Thomas Frieden, director of the CDC, said more than 2 000 people have been in contact with infected individuals, and only a handful have become ill.
Virtually all of the rest have had direct contact with poultry, the identified cause of the virus.
"This particular virus is not going to cause a pandemic because it doesn't spread person-to-person," Frieden said. "But all it takes is a bit of mutation for it be able to go person-to-person."
"I cannot say with certainty whether that will happen tomorrow, within 10 years or never."
The new strain of bird flu known as H7N9, which began infecting people in February, has so far sickened at least 127 people and killed 27. According to the latest CDC estimates, the flu kills about 20% of the people it infects.
The United States has been working closely with Chinese health officials, and has recently distributed test kits to detect this new strain of flu, which has never before appeared in humans.
Tests by Chinese health officials have found the virus in chickens, ducks and pigeons, but Frieden said it is not yet clear how the virus spreads in birds.
New strains of flu present a threat because if they do become easily transmissible, they might quickly spread around the globe, attacking individuals who have no natural defense against the virus.
The CDC has activated its Emergency Operations Center to monitor the disease and currently has 193 staff working on H7N9. "We've got a team working in China, Laos, Cambodia and Vietnam," he said.
Worrisome factors
Frieden said there are several factors that make this particular virus especially worrisome.
An analysis of the genetic code of the virus shows that it has receptors that bind to the lower respiratory tract of people, much like the more familiar bird flu strain known as H5N1. "That is why it's causing severe disease," Frieden said.
But it also has receptors that bind to the upper respiratory tract of people, which may explain why it is more transmissible from birds to people than H5 appears to be, he said.
And unlike H5N1, which caused severe disease in poultry, this new virus does not, which may make it more difficult to control because researchers will not be able to cull poultry flocks.
Frieden said even with H5, it took 18 months from the emergence of the virus until the 100th case. By comparison, it took only about one month from the emergence of H7 until the 100th case.
"If you look at the geographic spread of H5, within a couple of years it was all over Asia, into Africa, into the Middle East," he said.
Frieden said he cannot predict what the spread of H7 will be in birds, though he said he is concerned it may be quite wide.
"If there is evolution in the virus, it could go person-to-person, and that could cause severe pandemic."
In the United States, the CDC has developed and distributed H7N9 test kits, given to states and to several countries.
Frieden said the agency is working on flu vaccines for the virus and clinical trials could begin in the summer.
