The doses will be given 21 days apart, testing two vaccines from manufacturers Sanofi Pasteur and CSL Biotherapies. If there are early indications that the vaccines are safe, similar trials in healthy children (six months to 17 years) will begin, NIH said.

The H1N1 vaccines will be given to different sets of volunteers either before, after or at the same time as the seasonal flu vaccine.

'These data will be factored into the decision about how and if to implement a 2009 H1N1 flu immunization programme this fall,' said Anthony Fauci, director of the NIH's National Institute of Allergy and Infectious Diseases.

The World Health Organisation declared that the new virus, which surfaced mid-April, had caused a global pandemic, with more than 1.2 million cases reported so far.