GlaxoSmithKline said on Monday a single shot of its H1N1 swine flu vaccine protected people from the virus, adding to evidence that tight supplies can be stretched further by avoiding the need for two doses.

The British drugmaker is the latest company to report success with a single dose. Results from the first clinical trial with its candidate vaccine showed nearly 100 percent protection three weeks after vaccination.

A single dose of Glaxo's adjuvanted vaccine protected 98 percent of healthy volunteers, even though it contained a lower than normal amount of antigen, or active ingredient.

Adjuvants are immune-stimulating compounds added to vaccines to boost their effectiveness.

In fact, Glaxo's adjuvanted vaccine, containing 5.25 micrograms of antigen, performed better after three weeks than the an unadjuvanted one, with 21 micrograms, in the 130-patient trial conducted in Germany.

H1N1 swine flu vaccines would be given separately from regular seasonal flu shots, and pandemic sales to governments are expected to provide a sales windfall for many manufacturers this year and next.

Glaxo shares were 0.3 percent higher, outperforming a 0.4 percent fall in the European healthcare sector by 1120 GMT.

"This trial provides encouraging data on the potential use of a single dose of our pandemic vaccine," said Glaxo vaccines head Jean Stephenne.

The final formulation of Glaxo's adjuvanted vaccine is expected to have just 3.75 micrograms of antigen.

Because it is a new strain, infectious disease experts had initially anticipated people would likely need two doses of any H1N1 vaccine, given three weeks apart, to get full immunity against swine flu.

But US trials of Sanofi-Aventis's and CSL's H1N1 vaccines showed last week that only one dose was needed. A week earlier, Novartis and China's Sinovac also said they could protect people with a single dose.

The new H1N1 strain of flu, declared a pandemic in June, could eventually infect one third of the world's population, or 2 billion people, according to the World Health Organisation.

Glaxo -- which is one of the world's largest makers of seasonal flu shots -- is conducting a further 15 studies in over 9,000 subjects, including healthy adults, the elderly and children across Europe, Canada and the United States. - Reuters