Dr. Mark Adler receives swine flu vaccine nasal mist at Children's Memorial Hospital in Chicago on Monday. (David Banks/Associated Press)

The director of the Centers for Disease Control and Prevention in Atlanta tried to downplay fears about the H1N1 vaccine being fast-tracked, unnecessary or too late.

"It's too soon to say it's too late because we've don't know what the rest of the season will bring," Dr. Thomas Frieden told reporters on Tuesday.

Even in areas of the U.S. where the swine flu virus is widespread, about five to 10 per cent of the population have been affected, meaning 90 to 95 per cent are still susceptible.

The U.S. has not seen a flu season like this one in at least 50 years, Frieden said, calling H1N1 "no small matter" that has killed children.

But no corners have been cut in getting the vaccine ready, and it is being made and tested in the same way as previous flu vaccines, he said.

Canadian swine flu shots in November

The U.S. started administering a nasal spray version of the swine flu vaccine on Monday to health-care workers, school children and people caring for infants who cannot be vaccinated themselves.

The first doses of the injectable form of the vaccine are expected to start flowing in the U.S. next week, and will be focused on a wider group, including pregnant women.

"The flu vaccine is our best weapon against the flu," Frieden said, noting that adverse reactions will be carefully tracked.

China and Australia have also started offering pandemic flu vaccines

Health Canada said Tuesday that it is still on track to start offering the H1N1 vaccine in early November, based on clinical trial data from Europe.

Swine flu has hit earlier this fall in the U.S. than in Canada. The disease is on the upswing in some states and decreasing in others.

For the week ending Sept. 29, a map from the Public Health Agency of Canada showed widespread flu activity in southern British Columbia, with localized activity in the Northwest Territories and New Brunswick.

There was sporadic activity elsewhere, with no activity in Manitoba and northwestern Ontario.