Canada's health minister, Leona Aglukkaq, approved the H1N1 vaccine Wednesday as boxes of the vaccine are stockpiled in provincial and territorial warehouses.

Two million doses of the vaccine have already been shipped to the provinces and territories. The vaccine needed regulatory approval before the federal government could give the green light to start the flu shots.

Canadian clinical trials are now underway, but the results won't be available until next year. So federal health authorities are relying on data from clinical trials done on the same vaccine in Europe.

Those trials have found the vaccine safe in adults. The Canadian clinical trials are focused on determining the safety and effectiveness of the vaccine in select population groups, such as First Nations, people who are HIV positive, children and pregnant women.

Health officials have ordered 50.4 million doses of the H1N1 vaccine, which contains an adjuvant additive, a compound that will make it more effective. Another 1.8 million doses of the vaccine without the adjuvant are also on the way.

This means all Canadians can have access to the vaccine, unlike in the United States, which has only about 40 million doses for a population of 300 million.

Canada has never approved an adjuvant-containing flu vaccine before, so regulators have wanted to be assured of its safety before approval.

Because some reactions from vaccines are so rare they arise in only one in a million cases, the final picture of the vaccine's safety won't be clear until after the immunization program is underway

The vaccine without the adjuvant must be manufactured, packaged, and shipped separately. It's not known when the non-adjuvanted vaccine will be ready.

Dr. David Butler-Jones, Canada's chief public health officer, says that while pregnant women are encouraged to receive the vaccine without the adjuvant, the vaccine with adjuvant is still safe for expectant mothers.

While there have been few studies of adjuvanted flu vaccine on pregnant women, the risks are only theoretical, whereas the risk of swine flu to pregnant women are real and significant, Butler-Jones has said.

"The risks to pregnant women from contracting H1N1 are much, much higher than any theoretical risk posed by adjuvanted vaccine," he said earlier this week.

About 4,700 people worldwide have died of H1N1 to date, including 83 deaths in Canada.