The egg-based method is vulnerable to contamination -- the cause of a massive shortage of flu vaccines in the U.S. in 2004. Cell- based vaccines are thought to be less vulnerable.

The makers of the experimental vaccine, Protein Sciences Corp. of Meriden, Conn., altered an insect virus to produce a human version of a flu protein capable of sparking an immune response.

Like typical winter flu vaccines, the experimental vaccines used proteins from three flu strains picked annually by the FDA.