WASHINGTON, March 20 (Reuters) - A skin patch helped boost a bird flu vaccine so well that people appear to be protected by a single dose, researchers at biotechnolgy firm Iomai <IOMI.O> said on Thursday.

The so-called adjuvant patch, designed to be used with an injected vaccine, could help stretch the supply during a pandemic, the Maryland-based company said.

Current approved vaccines against the H5N1 avian influenza virus require two doses to be fully effective.

Iomai is testing its adjuvant patch on 500 volunteers in a phase 1/2 trial looking at the safety and efficacy of the patch. The patch, which is applied after gently scraping the skin with a light, sandpaper-like device, is being used to boost an H5N1 vaccine made by the Belgian drug company Solvay <SOLV.BR>.

When used with a single dose of the 45-microgram H5N1 vaccine, 73 percent of those tested had what is considered a protective immune response. About 49 percent of those who got the vaccine alone, without a patch, had an immune response considered protective after the first dose.

"We are thrilled," Iomai's chief scientific officer Gregory Glenn said in a telephone interview.

"The prospect of being able to immunize during a pandemic with a single dose is very attractive," said Glenn, whose company got a $128 million grant from the National Institutes of Health to test the patch.

The H5N1 avian flu virus is sweeping through flocks of poultry in Asia and sometimes in Africa and Europe. It has infected 373 people in 14 countries and killed 236 of them since 2003.

The fear is that the virus might change just enough to pass easily from one person to another, sparking a deadly pandemic.

At least 16 companies are testing H5N1 vaccines but no one knows precisely what a pandemic strain of the virus would look like or how to formulate the best vaccine. Tests on the current vaccines suggest that people need bigger doses than with seasonal influenza.

Global flu vaccine production capabilities are limited and if bigger doses are needed, that means fewer people could be vaccinated in a pandemic.

"A one-dose pandemic flu vaccine is a very important advance," Glenn said. "There is just almost no way to immunize twice in the face of a pandemic." Keeping the right records and counting people to show up twice are both barriers, he said.

Adjuvants are frequently used to boost vaccines and some of the experimental H5N1 vaccines include adjuvants in the formulation.

Iomai is working to use its needle-free technology to make vaccines against seasonal influenza and traveler's diarrhea. (Reporting by Maggie Fox, Editing by Michael Kahn and Bill Trott)

AlertNet news is provided by