TORONTO - Canada will purchase 50.4 million doses of pandemic flu vaccine, an amount that should be sufficient to protect all Canadians who want to be vaccinated against the swine flu virus, federal officials said Thursday.

The order was announced by Health Minister Leona Aglukkaq, who said the federal government would pick up 60 per cent of the $403.2 million tab for the vaccine.

"This investment reflects the unique circumstances of the situation as well as the gravity," said Aglukkaq, who called it a special one-time contribution. She noted that with seasonal flu vaccine purchases, provinces and territories are responsible for 100 per cent of the cost of their programs.

The vaccine will be bought from GlaxoSmithKline, which will make the product at its facility in Ste-Foy, Que.

Dr. David Butler-Jones, head of the Public Health Agency of Canada, said the size of the order should be sufficient to cover all Canadians who want and-or need pandemic vaccine.

"I think there's a fair degree of confidence that the amount that we've ordered will be sufficient," he said in an interview with The Canadian Press.

"Again, I'm waiting to see what the clinical trials show before being definitive."

That is a reference to the fact that it is not currently known whether one or two doses of vaccine will be needed to protect against the new H1N1 virus.

It is thought, though, that older adults -- who seem less vulnerable to the virus -- may be able to get by with only one dose. And Butler-Jones said because Canada is likely going to be using vaccine containing an adjuvant -- a compound that boosts its potency -- there is reason to believe one dose may suffice across all age groups.

He explained the federal math this way:

There are 33.6 million Canadians. The federal pandemic plan suggests authorities should bank on 75 per cent of Canadians wanting or needing vaccination. With 50.4 million doses, 75 per cent of Canadians could each get two doses.

But if only one dose is needed for some or all Canadians, different formulas could come into play.

The order size obviously allows for one dose for 100 per cent of Canadians, or one dose for all and a second dose for 50 per cent of the population, if studies show some people need two doses, Butler-Jones said.

Canada has the option of going back to request more vaccine if studies show two doses per person are needed and demand outstrips supply, Butler-Jones said. But he called that "a very, very unlikely situation."

Polling the government has commissioned suggests that at this point, only about 60 per cent of people might want pandemic vaccine. Only about a third of Canadians get a seasonal flu shot, Butler-Jones pointed out.

"We're ordering more," he said. "This is all hedging our bets to ensure that we err on the side of caution."

In fact, it is quite possible the country could find itself with excess pandemic vaccine on its hands, the chief public health officer admitted.

He said the government is talking with GSK about potentially turning back part of the order, if it becomes apparent Canada doesn't need 50.4 million doses.

"The most likely scenario is that we will end up with excess vaccine. So we'll be working closely with the manufacturer to make sure there are ways to ensure that that can get to other countries," Butler-Jones said.

Canada has first access to vaccine produced at the Ste-Foy plant, having signed the world's first pandemic flu vaccine contract in 2001 with Shire Biologics, a previous owner of the plant. GSK inherited the contract when it bought the facility in 2005.

GSK has indicated it wants to sell an adjuvanted vaccine and the World Health Organization has urged countries to use vaccine formulas that allow limited global supplies to be stretched as far as possible.

Canadian officials have repeatedly said they expect to start receiving supplies of the vaccine in late October or early November, by which time clinical trials already underway in other countries should inform decisions on the size and number of doses needed.

The WHO said Thursday that trials have started in China, Australia, Germany, the United States and Britain and more will start in the days to come.

The head of the agency's initiative on vaccine research, Dr. Marie-Paule Kieny said production of the pandemic vaccine is "on track" and some countries may start to administer vaccine as early as September.

Butler-Jones said Thursday that the public health agency expects Canadians who want vaccine will receive at least one dose before Christmas. If a second is required, those doses will likely be administered after Christmas, he said.

When two doses of a vaccine must be given, there has to be an interval between the doses. For this vaccine, it is expected the interval would be at least 21 days.