Kathleen Sebelius, the health and human services secretary, said that the eight to 10-day period between taking the vaccine and immunisation was quicker than initially expected.

Very few side effects to the vaccine were found, according to Fauci.

Only one dose of the vaccine was shown to be necessary in tests by CSL last Thursday to protect patients.

In other results released on Friday, trials in Australia showed the effectiveness of another vaccine in one shot, in 21 days.

China's Sinovac Biotech Ltd also said last week their H1N1 vaccine was effective in one dose.

At least 3,205 people have died internationally since the H1N1 virus first surfaced, with the first reported death being in Mexico last April.