TORONTO - A decision about which groups of Canadians should be front of the line to receive the swine flu vaccine once it becomes available won't be made until early to mid-September, federal health officials said Wednesday.

Unlike the United States, which is drawing up a priority list because vaccine supplies are expected to fall short of demand, Canada is on track to have enough of the agent to inoculate all Canadians who need and want it, said Health Minister Leona Aglukkaq.

"We don't need to make that decision today, as the production will be starting sometime in October," Aglukkaq said of the decision about who will be first to roll up their sleeves for a shot in the arm.

"We have a window of opportunity here to continue to learn more about H1N1 so that by the time we develop the priority listing that it is based on the best information that we have, based on the cases that we're seeing across Canada, as well as the global community," she told an Ottawa news conference.

Also on Wednesday, a U.S. government panel recommended that pregnant women, health-care workers and children six months and older be among those heading the priority list for swine flu vaccine. As well, those first vaccinated should include parents and other caregivers of infants; non-elderly adults who have high-risk medical conditions; and young adults ages 19 to 24.

The Advisory Committee on Immunization Practices (ACIP) voted to set vaccination priorities for those groups during a meeting in Atlanta. The panel's recommendations are usually adopted by U.S. federal health officials.

Ottawa has had a contract since 2001 with GlaxoSmithKline to provide enough pandemic vaccine for the entire population, if need be.

Aglukkaq said the company should be ready to begin clinical trials of its H1N1 flu vaccine by October, if not earlier. "Assuming all goes well and initial clinical trials indicate the product is safe and effective, immunization programs will most likely begin in November."

Dr. David Butler-Jones, Canada's chief public health officer, said experts want to collect as much information as possible, including data from clinical trials of vaccines in other countries, before making the decision about who should be first in line for vaccination.

He said a priority list would decide which groups -- health-care providers and pregnant women, for instance -- should get their shots once provinces and territories roll out their vaccination programs.

"In Canada, the issue of vaccine is not one of who will get vaccine or not, it's who will get it in the first week versus, say, the fourth week of the immunization program," he said. "So it's not a prioritization in the sense of most countries are prioritizing because they don't have enough vaccine for everybody, so they are choosing groups to be immunized first."

Dr. Donald Low, microbiologist-in-chief at Toronto's Mount Sinai Hospital, said in an interview that it's critical for the United States to have a priority list for those groups "recognized as at higher for more severe disease."

In fact, he said, the groups identified by ACIP account for about half the U.S. population.

"They're taking this relatively broad group probably expecting that they will get a relatively small uptake," he said. "It's hard to predict how anxious people are going to be to go out and get a vaccine when they've heard that this is such a mild disease."

In Ontario, for instance, about 40 to 45 per cent of people get inoculated against seasonal flu, which kills an estimated 2,000 to 4,000 Canadians in any given year. So far, H1N1 has been linked to 58 deaths.

While most swine flu cases in Canada continue to be mild, Butler-Jones agreed some Canadians are at higher risk than others for developing severe disease and they will be high on the list for immunization.

Among those at greatest risk are pregnant women, whose immune systems are altered and their response to infection increasingly blunted as pregnancy progresses.

In a study published Wednesday in The Lancet, researchers at the U.S. Centers for Disease Control and Prevention reported that pregnant women who get swine flu are at least four times more likely to be hospitalized than other people with the virus.

Their analysis of the first 34 cases of H1N1 flu in pregnant women in the U.S. between April and mid-June showed six died after contracting the infection. Pregnant women have accounted for an estimated six per cent of U.S. swine flu deaths since the pandemic began in April, even though they make up just one per cent of the population.

Among 60 Canadian women aged 15 to 50 who were admitted to hospital ICUs with severe disease from swine flu, nine (or 15 per cent) were pregnant, and two of them died.

So when it comes to a priority list for Canada, said Butler-Jones, "pretty clearly pregnant women, because of the risk, are one of the groups we would hope to immunize early on."

Still, some women may be fearful about exposing their developing fetus to any kind of drug, especially a new vaccine.

Low said studies of previous vaccines have shown no danger to pregnant women or their fetuses and most public health agencies around the world recommend their use even when a woman is expecting.

"The message has to go out to pregnant women that we have a long history of use of influenza vaccines in pregnancy," he said. "What we do know is H1N1 is a great risk to a pregnant woman if she becomes infected."

Butler-Jones said that with the pandemic flu strain, at least 35 per cent of Canada's population is expected to become infected -- and that will include pregnant women.

"And we know that influenza in pregnancy is a nasty disease. So you're trading off the ability to prevent a very nasty disease against a small theoretical risk of something that might happen if you got the immunization."

--With files from The Associated Press