American health officials said Thursday it’s too early to determine if a "one shot" H1N1 flu vaccine will be effective.

Both China and the Swiss pharmaceutical company Novartis have approved a vaccine they say prevents the new flu in a single dose.

The World Health Organization said it is encouraged after reviewing the test details from the vaccine by Beijing-based drug maker Sinovac Biotech Ltd.; the WHO would not comment on Novartis' vaccine since it had not yet seen the data. It said a number of companies were working on one-dose formulations that could theoretically increase the world's swine flu vaccine supplies.

But the U.S. was taking a wait-and-see position.

“There’s not much we can say at this point,” said Patricia El-Hinnaway, a spokeswoman for the U.S. Food and Drug Administration. “We’re waiting for data to come in from the clinical trials.”

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.

Novartis said its results were based on a British trial of 100 people between 18 and 50 years old who received either one or two shots of its swine flu vaccine. It is also testing other swine flu vaccine formulations in more than 6,000 people worldwide.

According to Novartis, people who got two injections had a better immune response, but just one injection provided adequate protection within two weeks.

It is still unclear what this may mean for the global vaccine supply. Not every drugmaker could simply copy Novartis' vaccine recipe: Novartis' vaccine was made using cell culture, while about 90 percent of the world's flu vaccines are made using chicken eggs.

Their vaccine also includes adjuvants, chemical components used to stretch a vaccine's active ingredient and essentially make it more efficient. While adjuvants are commonly used in European flu vaccines, there are no licensed flu vaccines in the U.S. or Canada with the ingredients.

There is also limited data on how safe flu vaccines with adjuvants are in pregnant women and children, two of the groups believed to be most vulnerable in the H1N1 pandemic.

The vaccine approved by China did not use an adjuvant, like most of the vaccines being tested in the U.S.

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

The Associated Press contributed to this report.