The Geneva-based World Health Organization says the H5N1 influenza virus is the most likely candidate to cause an avian influenza pandemic, provided it mutates rapidly and has already showed its ability to infect humans.

The vaccine under study is fractionated, meaning it only contains parts of the viral proteins that activate the human immune system and therefore cannot cause influenza. But it does contain an adjuvant that enhances immunogenic effects, allowing a reduced concentration of vaccine in each dose.

That would mean both vaccine production and the number of available doses could be increased in the event of a pandemic.

Officials say 5,052 volunteers will participate in the study. They will be randomly separated into two groups — 75 percent who will receive two doses of influenza vaccine, and 25 percent who will receive a dose of conventional influenza vaccine and a dose of placebo.