If they're right, it would be good news. Many health researchers fear it will take two shots to protect people, vastly complicating efforts to stem the spread of the illness.

The World Health Organization says it is encouraged after reviewing the test details from the vaccine by Beijing-based drug maker Sinovac Biotech Ltd. — one of several being developed in China. However, experts said more results are needed from other vaccine makers to determine if one dose would be potent enough.

Australia-based CSL should know within days whether one dose of its vaccine, administered to volunteers in that country in late July, was enough. CSL to date has been mum.

In about two weeks, the U.S. expects to announce initial test results from its vaccine, which is the same type as one of the Chinese versions, said Dr. Anthony Fauci of the U.S. National Institutes of Health.

"From what I've seen and heard of the data, it looks encouraging," Fauci said of the clinical trials of Sinovac vaccine. "This is very good news. Let's hope the material that we're using has similar results."

Most experts agreed.

"Everybody is desperately hoping that one (shot) will do because then that's much easier to administer," said Jodie McVernon, a vaccine expert at the University of Melbourne who is involved in Australian trials of swine flu vaccines for young children. She had not seen the Chinese trial results.

However, James McGlothlin, a member of Purdue University's pandemic planning committee, was cautious about the Chinese report.

"They've got some very good scientists over there, but anything that sounds too good to be true ought to be scrutinized," he said in a telephone interview.

"I'd like to look at some of the clinical trials," that led to the one-dose conclusion, he said. "In China, the rules are a little bit different in terms of human subjects," and it's not clear what safety factors were in place, he said.

China's State Food and Drug Administration said Thursday it approved the vaccine by Sinovac, which completed testing last month.

The vaccine is the first to be approved by the Chinese regulator and is set to be followed by at least one other vaccine this week, made by Hualan Biological Engineering Inc. Another four vaccines were being reviewed, the regulator has said.

Both companies say their studies show one shot of vaccine is effective on people ages 3 to 60. More than 3,000 participated in the trials.

Sinovac says it has the capacity to produce up to 30 million doses of swine flu vaccine in a year, while Hualan said it can make 160 million doses.

Stockpiling vaccines is China's latest move in its aggressive approach to contain the spread of swine flu in the country of 1.3 billion people and relatively limited medical resources. It has quarantined travelers on suspicion of contact with infected people and ordered schools to test students' temperatures.

The Health Ministry says nearly 4,000 cases of swine flu have been confirmed on the mainland — none fatal.

China aims to have enough swine flu vaccine for 5 percent of the public by the end of the year, and although health officials have not released detailed vaccination plans, they have said health workers, public service workers and students are priority groups.