The product is already marketed in Europe, Australia and Canada by Sanofi Pasteur, the vaccine division of French drug giant Sanofi, previously known as Sanofi-Aventis SA.

Most of the world's flu vaccine is a killed virus, given as an injection; Europe's Sanofi Pasteur and Novartis are leading producers that use this approach.

Indeed, a spokesman for Sanofi's own vaccine unit, Sanofi Pasteur, says it could probably produce both types "in parallel" for a time, before ramping up full-scale production of a swine-flu vaccine.