By Maggie Fox, Health and Science Editor

WASHINGTON, Aug 16 (Reuters) - A new additive has allowed doctors to stretch an experimental GlaxoSmithKline <GSK.N> bird flu vaccine, offering some hope of being able to vaccinate more than just a few people in case of a pandemic.

The new additive, called an adjuvant, allowed a dose one-quarter the size of that used in the annual seasonal flu vaccine, the company-funded researchers reported in Friday's issue of the Lancet medical journal.

The approach could allow a limited vaccine supply to be stretched up to 25-fold, meaning as many as 25 people could be vaccinated with a dose that would normally only protect one person, outside flu vaccine experts said.

"These findings are important to anyone involved in pandemic preparedness because the number of prepandemic vaccine doses can be stretched 20 to 25-fold, when compared to the ... dose required by the current A/H5N1 vaccine approved by the U.S. Food and Drug Administration," Dr. Suryaprakash Sambhara and Dr. Gregory Poland of the U.S. Centers for Disease Control and Prevention said in a commentary.

Experts agree a pandemic of some sort of new influenza virus is possible at any time. No one can predict when or what strain, but the H5N1 avian influenza virus, which has infected 321 people and killed 194 of them, is the chief suspect.

It could infect and kill millions and a vaccine would be the best defense.

So at least 16 different companies in 10 countries are trying to make vaccines against H5N1. Because making influenza vaccines is uncertain and time-consuming, researchers want to find ways to stretch the few doses now available.

Geert Leroux-Roels of the Centre for Vaccinology at Ghent University in Belgium and colleagues tested various doses of Glaxo's experimental H5N1 vaccine in 400 adults.

The vaccine uses a new adjuvant called ASO3, which stimulates the immune system with oils, water and detergent.

They tested the blood of their volunteers after injection and found the smallest dose stimulated what is considered a protective immune response in more than 84 percent of them.

And while the vaccine was formulated using one of three sub-strains -- called clades -- of H5N1, the protection seemed to extend to the other clades.

Such so-called cross-immunity is important because influenza viruses mutate quickly and no one knows what changes may occur between now and the time a pandemic strain of virus breaks out.

The vaccine, like most flu vaccines, uses a single protein from the flu virus called hemagglutinin. This gives a flu virus the "H" part of its name, and for some reason H5s do not stimulate much of an immune response in humans.

That is why using an adjuvant is so important -- to ensure the vaccine provides enough of an immune response to protect people against infection.

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