TORONTO — The current wave of H1N1 activity may have peaked in all provinces and territories, the Public Health Agency of Canada says in its latest assessment on the state of the pandemic.

But if transmission of the virus is indeed on the downward slope, does that mean the end is in sight for Canada? Or is a third wave of illness sometime this winter in the cards?

With the notoriously unpredictable influenza, it’s pretty much anyone’s guess.

“I think if you ask a group of so-called influenza experts, you’re going to get different views,” says Dr. Frederick Hayden, an influenza expert who splits his time between at the University of Virginia and Britain’s Wellcome Trust, a charity that funds biomedical research.

Hayden is betting on “a bump” of activity after Christmas, because so many people remain susceptible to the virus. Though whether that will be a full wave or just an upswing in cases, he’s not sure.

Regardless of where the experts come down on the question of a third wave, they say one thing is a safe bet. Whether it’s this winter or next, the H1N1 flu will be back.

That’s something people vacillating over the value of getting vaccinated at this point should keep in mind, says Dr. Danuta Skowronski, an influenza epidemiologist at the British Columbia Centre for Disease Control.

“The way I see it, basically people have two choices in general: Get immunized or get infected,” she says. “Because this virus isn’t going anywhere. It’s ours now.”

If this outbreak follows the pattern of previous pandemics — and there’s no reason to believe it won’t — the pandemic virus and its descendents will be among us for some undefined period of time. It could be years, it could be decades.

In past pandemics, a new influenza A virus replaced the previous virus, making it the seasonal flu A strain until the next pandemic occurred. (Influenza B viruses also cause human illness but they don’t trigger pandemics and aren’t affected by the interplay between influenza A viruses.)

Going into this pandemic there were two seasonal A viruses, H3N2 and a seasonal H1N1 virus and it’s too soon to say whether the pandemic H1N1 will replace both. But there is every reason to believe the pandemic H1N1 will cause illness next winter and the winter after that.

The burning question at this point, though, is what will pandemic H1N1 do over the next few months?

Flu season doesn’t usually start in our neck of the woods until late November or December at the earliest, with activity generally spiking in January or February. Is it possible we could face a winter where we got most of our flu activity out of the way before Christmas?

“There’s a lot of winter left to go, and I think that’s the big unknown,” says Dr. Anthony Mounts, a flu expert with the World Health Organization.

Pandemics don’t follow a single pattern so there’s really no way to predict. But experts are looking to the most recent pandemics, 1957 and 1968, to try to get a sense of how this winter will play out.

Dr. Walter Dowdle worked on influenza at the U.S. Centers for Disease Control during the former and ran the CDC’s virology division during the latter. He expects to see some ongoing transmission of the new virus, but not the sharp spike of activity seen in some places in the spring or early summer and seen in most places this fall.

Dowdle says we could even see “a peak here and there after Christmas.” Places that didn’t have as much illness as others likely have more “susceptibles” — people still vulnerable to the disease. “And for places that haven’t had a lot of transmission you could very well see some uptick,” he says.

How many susceptibles there are out there is a question without an answer at this point. Some people estimate, however, that between the people who have already had H1N1, have been vaccinated or had pre-existing immunity (seen in some people over age 60), Canada is nearing the point where about half of the population is likely protected against the virus.

That still leaves a lot of people, says Dr. Allison McGeer, who believes we face a third wave this winter.

“I don’t think the pandemic strain is finished. I think there are too many susceptibles. I think it’s coming back,” says McGeer, head of infection control at Toronto’s Mount Sinai Hospital.

Dr. Arlene King, Ontario’s chief medical officer of health, frankly wishes people would stop spending so much time debating whether we’ll have another wave of H1N1 flu this winter.

“I’m concerned that the fixation on waves is distracting from the task at hand, which is to try to get as many people immunized as possible as quickly as possible,” says King, who was the Public Health Agency of Canada’s point person for pandemic preparedness for years until she took the Ontario job last spring.

“The more people we get immunized and clearly the more quickly we get them immunized, the more cases we’re going to prevent overall,” she adds. “Maybe we can stop talking about a third wave if we get enough vaccine into the population.”

The Canadian Press

hat-tip VA_MOM