Canada's chief public health officer is defending Ottawa's decision not to halt production on seasonal flu vaccine to accelerate production of the vaccine against swine flu.

Dr. David Butler-Jones says the decision to finish off seasonal flu vaccine production before beginning to produce swine flu vaccine was made months ago and was based on the information officials had at the time.

"The WHO asked all manufacturers to finish off their seasonal vaccine first. That's because in Canada, 4,000 people die on average every year from seasonal flu," he explained to CTV's Question Period on Sunday.

He noted that at the time of the decision, his team of experts looked at the flu's activity in the southern hemisphere, and what they saw was both seasonal flu strains and the swine flu strain circulating widely and causing deaths.

So, he said, they decided not to put the brakes on seasonal flu vaccine production to ensure that those Canadians vulnerable to seasonal flu -- generally, those over the age of 65 and well as those with compromised immune systems -- would be protected.

"The problem is if you switch over -- and we're talking about a week or two, we're not talking about months of difference -- if you switch over to H1N1, stop the seasonal flu, you can't go back," Butler-Jones said.

"So that was the decision at the time, and if we could do things -- I mean if the pandemic was started somewhere else, if we'd known about it years in advance or months in advance -- or we'd known what we knew today back then, then I think we would be further ahead than we are now. But you could never have known that," he said.

He noted that in the U.S., emerging evidence suggests that 10 per cent of the flu strains circulating are seasonal flu strains, so those vulnerable to seasonal flu are still at risk.

"It's not an either/or equation. It's not like you can protect against one and nothing else will happen," he said.

While there have been swine flu clusters in British Columbia, the Northwest Territories and elsewhere, Canada is not seeing the large outbreaks being seen in the southern United States, he noted.

And although it will still be a few weeks before the swine flu vaccine is available, when it does arrive, Canada will be uniquely positioned to offer it to every citizen who wants it, he said.

"We know we can start the immunization program by the beginning of November. We will then rapidly be able to immunize everybody in this country," he said.

"By the time Christmas comes around, our hope is, and our expectation is that every Canadian who wishes to be immunized will be," he said, adding that every Canadian should get the vaccine -- if not to protect themselves, then to protect those around them at risk.