The vaccine - to be produced by Aventis Pasteur and Chiron - is a hybrid, comprising a flu strain that is used in standard influenza vaccines and genes taken from a strain of the H5N1 bird flu isolated from a Vietnamese patient last year.

Ultimately, two million doses would be stored for future use.

If a pandemic does break out, the new vaccine would be distributed to protect US laboratory workers and public health personnel. Given the limited stockpile, the government is seeking suggestions as to who else should get the vaccine in the event of an outbreak.