Health Canada is poised to approve the swine flu vaccine as early as Wednesday as Canadian tests on it get underway.

Worldwide trials of the H1N1 influenza A vaccine began in September, mostly on small groups of adults.

Canadian vaccine trials started this week, and the results won't be available until next year.

In the meantime, Canada's health regulator is relying on worldwide experience with producing flu vaccines and small European studies to approve the use of the vaccine in Canada.

"We're able to draw on results from the clinical trials to inform and expedite our regulatory review," said Dr. David Butler-Jones, Canada's chief public health officer. "And the development of the vaccine is proceeding."

Flu vaccines are safe and effective, but there is still a lot to learn about them, said Dr. Allison McGeer, an infectious disease specialist at Mount Sinai Hospital in Toronto.

"There's 50 years of using them; there's a lot we do know about them," said McGeer. "But every year, there's a little bit of a difference. How much information do we need? How much should we get to make sure [they're] safe and effective?"

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

The final picture of how the vaccine works won't be clear until well after Canada's largest immunization program is underway.