Ontario's priorities for offering seasonal and H1N1 vaccines varies by age. (John Amis/AP Photo)

People in Ontario who are over 65 and those living in long-term care facilities will be offered seasonal flu shots starting in October, followed by H1N1 shots for others a month later, the province's chief medical officer of health said Thursday.

Dr. Arlene King said the decision was made based on several factors:

• Emerging, unpublished evidence from Canadian studies suggesting a possible association between prior flu vaccination and getting the H1N1 pandemic flu virus.
• The latest science on the H1N1 virus and how it is behaving.
• The logistics of administering multiple vaccines.

"We're not dealing with a typical flu season this year," King told a news conference in Toronto.

"Seniors are most at risk for complications from seasonal flu while younger people are more at risk for complications from H1N1 flu," the province said in a release.

In November, Ontario will start to offer H1N1 flu shots to everyone who wants and needs it, following federal guidelines on who should be first in line, namely:

• People under age 65 with chronic conditions.
• Pregnant women.
• Healthy children age six months to under five years.
• People living in remote or isolated communities.
• Health-care workers involved in pandemic response or providing essential services.
• Household contacts and those providing care to those who can't be immunized, such as infants under six months of age or people with weakened immune systems.

Seasonal flu shots

After H1N1 vaccinations have wrapped up, people under 65 will be offered the seasonal flu vaccine, King said.

Other provinces and territories are also developing their vaccine rollout plans.

"We have to be very nimble and flexible based on the fact that we're dealing with a new virus, we're dealing with an emerging outbreak that we need to monitor and adjust our plans accordingly," Dr. Joel Kettner, Manitoba's chief medical officer, said from Winnipeg.

The preliminary Canadian research is a consideration, said Dr. Perry Kendall, British Columbia's medical officer of health.

"That has clearly been part of the debate," Kendall said. "Obviously if this is a valid, robust finding, it does impact to whom and how we'd like to deliver vaccines."

Ontario has 19.5 million doses of H1N1 vaccine, enough to treat 75 per cent of the population if two doses are needed. Clinical trials are ongoing to determine if one or two doses are needed to protect adults and children.

King said the province also has enough antivirals such as Tamiflu to treat 25 per cent of its population. Antivirals work best if given in the first 48 hours after symptoms appear.

People are also urged to continue to wash their hands frequently, practise other hygiene measures and stay home when sick.

Mixing vaccines

In another development on Thursday, the World Health Organization announced that a panel of experts will look at the unpublished Canadian data that suggests previous seasonal flu shots may put people at greater risk of catching the swine flu virus.

The UN health agency has already contacted researchers worldwide to try to corroborate the preliminary Canadian findings, but the effect has not been seen elsewhere, Marie-Paul Keany, WHO's vaccine research director, told a teleconference with reporters from Geneva.

She praised the Canadian investigators as well known and credible, but noted the new data needs to be reviewed closely.

Vaccine for developing countries

WHO also announced Thursday that pharmaceutical companies are short about three million doses of the H1N1 vaccine, much less than the five million doses the agency previously expected.

WHO's priority is to vaccinate at least health-care workers in every country, which is about two per cent of the world's population of 6.8 billion people.

So far, nine countries including the United States have said they would donate about 10 per cent of their H1N1 vaccine supply to developing countries, and vaccine manufacturers have also said they will donate 150 million doses.

Canada has not offered up any H1N1 vaccine, but Keany said WHO hopes Canada will be able to donate some serum.

The agency urged countries to continue to monitor the vaccinated for possible side-effects. So far, the side-effects are similar to those of seasonal flu shots, such as soreness, swelling, redness, fever, headache and muscle or joint aches.

"In almost all vaccine recipients, these symptoms are mild, self-limited and last one to two days," WHO said.

With files from The Associated Press