PRINCE.EDWARD.ISLAND (CBC) - A single dose of the swine flu vaccine without adjuvant seems to produce a good immune response in healthy pregnant women, but not for children under age 10, two U.S. studies suggest.

The research focused on the non-adjuvant form of the vaccine doses without a compound that boosts immune response. Most Canadians are being offered the vaccine containing the adjuvant.

The Canadian government has ordered 1.8 million adjuvant-free doses mainly for use in pregnant women, and health officials plan to start rolling out those shots this week.

The U.S. National Institute of Allergy and Infectious Diseases sponsored the latest studies on the vaccine. Pregnant women in the study tolerated the adjuvant-free vaccine well, and no serious safety concerns arose, said Dr. Anthony Fauci, the head of the institute.

So far, 28 pregnant women have died in the United States from swine flu, he said.

"Children six months to nine years had a less robust immune response," Fauci said.

The researchers said more study is needed to confirm that children need two doses to gain full protection.

On Friday, Canada's chief public health officer, Dr. David Butler-Jones, said it's recommended that young children receive two doses of the H1N1 vaccine with the booster compound. But federal health officials are revising the recommendation based on early data from another study that suggests a single dose of the adjuvant vaccine may suffice.

In both the U.S. and Canada, people have been lining up to be immunized.

The U.S. originally anticipated at least 80 million doses would be delivered to state health departments.

"Over time, we expect that supply will start to increase and eventually catch up with the tremendous demand that we are seeing now," Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention told reporters.

"As of today, 30 million doses of the H1N1 vaccine are available for the states to order," up from a cumulative total of 26.6 million doses of vaccine available on Friday.

On Monday, a team of non-government experts started looking at reports on the vaccine's safety from trials conducted by the government and vaccine manufacturers.

The panel is meant to spot quickly any potential rare problems with the vaccine and to explain false alarms of common conditions such as heart attacks that happen to occur at the same time the shot is given. About 2,500 miscarriages and 3,000 heart attacks occur every day in the U.S. normally.

"Given the rapidity with which this particular vaccine was rolled out, there seems to be an extra special obligation to make sure things remain as uncomplicated as they have in the past," Dr. Marie McCormick of the Harvard School of Public Health, who chairs the working group, told The Associated Press.

Initial reports to a U.S. government database where anyone can report any symptom, and serious ones get intense investigation showed nothing unusual after the first 10 million vaccinations, said Dr. Bruce Gellin, head of the National Vaccine Program Office.