Ontario says it is changing its approach to seasonal flu vaccine administration this year in part because of unpublished data that suggest the seasonal shot might raise the risk of catching swine flu.

Dr. Arlene King, the province's chief medical officer of health, says people 65 and older, who aren't at great risk of catching swine flu, will be offered seasonal flu vaccine in October.

But the seasonal flu won't be offered to younger people in Canada's most populous province until after they are first offered the vaccine for swine flu, which is likely to happen in November.

King says the unpublished study was clearly a factor in the decision to change the sequence of the vaccination program.

That study, based on research ongoing in British Columbia, Ontario and Quebec, suggests that people who have received seasonal flu shots last year may be at greater risk of catching H1N1 flu this year.

While the study is still being peer-reviewed in an unnamed journal, King said it had a direct influence on the policy for this year's flu vaccination program in Ontario.

"There is emerging unpublished evidence that suggests an association between prior seasonal flu vaccination and getting the H1N1 flu. The nature of the association is unclear at this time, so we want people under 65 to wait to get the seasonal flu shot until after the H1N1 flu vaccine until we learn more about this," she told reporters.

The other factors influencing the plan are evidence from the southern hemisphere that shows that H1N1 is dominating other seasonal flu strains.

The vaccines are also being phased in because of the difficulties of trying to provide both types of shots at the same time. That's because the seasonal flu vaccine will be ready in October but swine flu vaccine won't be ready until mid-November.

She added that there is still no scientific data that giving both swine flu and seasonal flu shots at the same time is either safe or effective. So Ontario will take a cautious approach and phase in both flu vaccine programs over three phases:

1. In October, the seasonal flu vaccine will be offered to Ontarians 65 years and over and residents of long-term care homes
2. H1N1 vaccine will be offered to the general population in November, following the vaccine sequencing guidelines issues by the Public Health Agency of Canada. That means high-priority groups first, and then the rest of the population
3. The seasonal flu vaccine will be then offered to all other residents six months of age and older who still want it.

"It's a different flu season this year and rolling out two flu vaccine programs is a great example of how different it is," Dr. King said.

Earlier in the day, the World Health Organization addressed the worries raised in the unpublished Canadian study in a telephone news briefing.

In a response to a question on the study from CTV's Avis Favaro, Marie-Paule Kieny, director of the WHO's initiative for vaccine research, said researchers in no other countries have made similar findings.

"We are in contact with other countries and having them look at their own data to see if they could have similar observations and none have been able to find anything like that," Kieny told the teleconference from WHO headquarters.

"So the reason why this may be different in Canada and in this particular study is not yet identified. It may be study bias; it may be that something is real.

"But the WHO is looking forward to be able to see the data and to study data and to come to a better understanding of whether this has any chance of indeed putting people are risk, the fact that they've received the seasonal vaccination," Kieny said.

Other provinces mull vaccine plan changes

Meanwhile, a number of Canadian provinces are considering suspending their seasonal flu vaccination programs, in part because of the worries.

Saskatchewan Health Minister Don McMorris said Thursday seasonal flu shots will likely be suspended for everyone in his province except seniors and those in long-term care.

Seniors are more susceptible to the seasonal flu, but their age group seems to be mostly immune from swine flu because of presumed exposure to similar viruses in their lifetimes.

McMorris said he was also worried about the reports that seasonal flu vaccine might make people more susceptible to swine flu. He added that other factors influencing their decision were expectation from flu officials that H1N1 would dominate this winter and crowd out seasonal flu strains.

In Quebec, health officials said earlier this week they're putting the seasonal flu shot program on standby. They too are considering either delaying seasonal flu vaccination until after swine flu vaccine programs are complete, or scrapping seasonal flu vaccination altogether.

"For the moment, it's on hold," Karine White, a media relations liaison with the Ministry of Health and Social Services in Quebec told The Canadian Press.

Swine flu vaccination programs likely wouldn't begin until mid-November, the earliest that the vaccine manufacturer will have it ready, tested and approved.