A single dose of Canada's vaccine against the H1N1 virus may be enough to protect adults, early results of a clinical trial suggest.

GlaxoSmithKline, which is under contract to produce 50 million vaccine doses for Canada, released the initial results of its first trial on the inoculation on Monday, based on tests on 130 healthy German volunteers aged 18 to 60.

The trial was designed to test the safety and effectiveness of the company's swine flu vaccine, which includes an adjuvant to boost strength and stretch supplies of the serum.

More than 98 per cent of subjects who received a first dose of 5.25 micrograms of vaccine and the company's AS03 adjuvant showed signs of protection three weeks later, compared with 95 per cent of those who received the vaccine without the adjuvant, the company said.

Lower-dose testing needed

But the study did not look at the dose GSK is using in the Canadian vaccine, namely 3.75 micrograms plus adjuvant.

A statement from GSK said the formulation tested was "comparable to the expected final formulation of the adjuvanted vaccine."

The company will also need to produce safety and effectiveness data for the smaller dose, said Dr. Allison McGeer, an influenza expert at Toronto's Mount Sinai Hospital.

"The truth of the matter is if you have good immunogenicity with a 5.25 mcg dose, then, yeah, probably 3.75 mcg is going to be not bad. But it's clearly better to have data with the real dose," McGeer said.

Last week, other researchers said studies suggest that 15 mcg of vaccine without adjuvant or 7.5 mcg with adjuvant are protective for adults.

The findings mean adults who wish to be vaccinated will have to roll up their sleeves twice this year instead of three times — once for a seasonal flu shot and again to be inoculated against swine flu.

It's thought that two doses of the swine flu vaccine may be needed for some populations such as children. Studies on children, teens, seniors and people with compromised immune systems are underway to test that theory.

With files from The Canadian Press