Countries around the world are preparing for a global outbreak of avian flu. Steven Riley, from the University of Hong Kong, says researchers are developing and stockpiling vaccines. The thinking behind this is that a very small mutation will eventually transform this avian strain into one that can infect humans.

The H5N1 influenza virus is always mutating, but the human vaccine is formulated and targeted against the current avian strain of the disease. As Riley explains, if you formulate the vaccine now, against the current avian strain, it will be available and ready to be used in the event of a human pandemic.

Even with this effort, scientists think there may not be enough vaccine for everyone, so a natural question arises: who gets vaccinated? The standard procedure is for certain high-risk groups to get the vaccine first: healthcare workers, emergency personnel, children and the elderly. Riley and his colleagues propose a radically different approach: use lower doses and vaccinate more people.

Their findings suggest that the proportion of people infected is likely to be reduced by giving a lower dose to more people, than by giving the maximum tested dose to fewer people. Riley estimates that by using a lower dose, and vaccinating 160, rather than 20, out of the 300 million people in the U.S., "you could be talking about 27 million fewer infections."

Riley arrived at these estimates by using a simple mathematical model. The results seem promising on paper, but he acknowledges that much depends on understanding the nature of protection. What does it mean to provide more people with only "partial protection," with a lower dose vaccine? "Does that mean that some people will have no chance of being infected despite many challenges?" asks Riley. "Or does it mean that they might just be protected for a couple of challenges and then get infected later in the epidemic?"

Riley himself admits that there are still some basic science issues to be resolved. Nevertheless, as researchers continue to develop vaccines, Riley hopes his study will give policy makers another, possibly more effective, option for protecting the public from a pandemic.