In the future, scientists will need to keep a closer eye on pig populations to spot similar emerging flu viruses, the researchers said in the Science paper.
In the United States, most cases of the swine flu continue to be no worse than seasonal flu. Testing has found that the swine flu virus remains susceptible to two common antiviral drugs, Tamiflu and Relenza, according to the CDC.
While the new swine flu only seems to cause relatively mild infection, experts worry that, if the virus mutates, people would have limited immunity to it. The CDC is concerned that, as the H1N1 virus moves into the Southern Hemisphere, where the flu season is just getting under way, it could mutate and return in a more virulent form in the Northern Hemisphere next fall.
On Thursday, U.S. health officials said that, while many states are still reporting new cases of infection, there seems to be an overall decline in visits to doctors and hospitals by people with the disease, indicating that the outbreak might be subsiding.
"At the national level, we're seeing that the percent of visits [to doctors and hospitals] for influenza-like illness is starting to turn down," Dr. Anne Schuchat, the CDC's interim deputy director for science and public health program, said during a teleconference. "That's a good sign. It's consistent with the idea that the worst may be over."
The CDC reported on Wednesday that some older people may have partial immunity to the new H1N1 swine flu virus because of possible exposure to another H1N1 flu strain circulating prior to 1957. So far, 64 percent of cases of swine flu infection in the United States have been among people aged 5 to 24, while only 1 percent involves people over 65, officials said.
On Friday, the CDC was reporting 6,552 U.S. cases of swine flu in 48 states, including nine deaths, although health officials said the death toll could be as high as 10.'/>"/>
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