Evan Tordorf, 4, gets his flu shot as his mother, Karen Joly, looks on at an H1N1 vaccination centre in Montreal on Nov. 6. (Ryan Remiorz/Canadian Press)

Public health officials are warning that Canadians shouldn't decide they can do without the swine flu vaccine just because they've made it this far without a shot.

A wave of infection appears to be peaking in some parts of the country and is perhaps past its peak in others, but the virus still could have plenty up its sleeve, they say as delivery of H1N1 vaccine to the provinces ramps up this week after an unfortunate slowdown.

In parts of the southern United States where flu activity had started to decline there are signs another upsurge in infections is coming, influenza epidemiologist Lone Simonsen said Sunday.

Simonsen, a researcher at George Washington University in Washington, D.C., has done extensive study of the wave patterns of previous pandemics.

"In '57 … we certainly had a fall wave followed immediately by a winter wave. And then we had the same pattern in 1918," she says, adding that in the pandemic of 1889, the bulk of the deaths occurred in the third wave.

"It would not be surprising to anyone if there was another wave here this winter."

New estimates posted online Sunday by the Public Health Agency of Canada said 1,993,800 doses of vaccine will be shipped to provinces this week, just under half of which will be without adjuvant. The adjuvant-free product was purchased for use in pregnant women.

Vaccinations too late?

Critics of the way the vaccine program has been handled have said the vaccine is coming too late. Dr. Richard Schabas, a former chief medical officer for Ontario, said last week that H1N1 will be a "distant memory" by Christmas and there will be no value in immunizing healthy people.

His view isn't shared by Dr. Alain Poirier, Quebec's chief public health officer.

'I don't know why this is, but I know that it happens more often than not that you'll have more waves in the same season when you have a new and emerging pathogen like that.'— Lone Simonsen

"People are saying the vaccinations are being done too late," Poirier said late last week. "But every person vaccinated is another person protected."

"Often there's a third wave and we could have that in February or March — we don't know."

Seasonal flu viruses don't generally have more than one peak (per virus subtype) per season. But Simonsen says history shows pandemic viruses break the rules as they are getting used to their new human hosts.

Multiple pandemic waves

"I don't know why this is, but I know that it happens more often than not that you'll have more waves in the same season when you have a new and emerging pathogen like that," she says.

The other point to consider, Simonsen suggests, is that the virus has hit different areas at different times with different levels of intensity. While Toronto is clearly in a second wave, some places didn't have much activity in the spring. For them, this isn't a second wave, it's a first.

"You can't talk about a national pattern when you have such big countries as we have here, Canada and the United States," she says. "I think it's more heterogeneous than I would have thought."

Another benefit of vaccination now is the fact it can take several years for pandemic viruses to settle down to the point where they act like seasonal viruses, meaning they stop taking such a heavy toll on younger people and shift their focus to their traditional target, the elderly, Simonsen said.

"For 1918, for example, it took a good three-four-five years before it got back to business as usual," she said. "It takes a little while before it settles in."

Whereas with seasonal flu — which mutates constantly — last year's shot may not be much good against this year's virus, that's not necessarily the case in a pandemic.

Because the virus still has plenty of people it can infect, it isn't under pressure to mutate. And so far, very little change has been observed. After the pandemics of 1957 and 1968, it took several years before the viruses mutated to the point that the vaccine had to be updated.

The time frame to change could speed up if hundreds of millions of people around the world are vaccinated against the virus. Still, people vaccinated this fall could be protected for awhile — especially in Canada where the vaccine contains an adjuvant which should protect more broadly against mutated viruses.

Seasonal comparison 'apples and oranges'

"I don't think you are done being protected by this because you got it slightly after the wave peaked," Simonsen says.

But Schabas, chief medical officer of health for Hastings and Prince Edward Counties in eastern Ontario, has insisted the H1N1 outbreak is not a major public health event.

Schabas told the media last week that once the "dust has settled on H1N1" between 200 and 300 people will have died in Canada — far fewer than those who die from seasonal flu every year.

Simonsen says this comparison is "apples and oranges" and doesn't take into consideration the fact that the people who are dying from this flu are more likely to be young adults than people in their 70s and 80s.

"If you count years of life lost instead of lives lost, you will see that it's a lot different to have a mean age of 75 years in those who die and a mean age of 30," she says.

"It is a different texture to have a lot of children and young adults dying than people who are nearing the end of their natural life cycle."