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A registered nurse injects a dose of the H1N1 flu vaccine at a Toronto health clinic on Thursday, October 29, 2009.

Darren Calabrese

Public health officials are warning the wave of H1N1 this fall is not nearly as widespread as suspected, leaving a significant number of Canadians who believe they have gained a ticket to immunity still vulnerable to the virus.

Last week in Ontario, fewer than one in four cases of suspected H1N1 tested positive in laboratory analysis. In British Columbia, the rate was one in three. The rest, based on samples, were mostly rhinovirus - the common cold - and enterovirus, a common flu-like bug.

In a year when Canada has collected record amounts of clinical data about influenza, the evidence is mounting that even doctors aren't very good at telling the difference between H1N1 and the common cold.

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"We have been notoriously inaccurate," said Michael Gardam, director of infectious disease prevention at Ontario's Agency for Health Protection and Promotion.

"This year is very different because we are swabbing way more than we ever would and we are more accurate than we've ever been before - and it's still not very accurate."

With labs overwhelmed by the number of cases being sent for testing, health officials are now asking doctors not to bother testing for H1N1 except in certain circumstances, such as for pregnant women or other people with underlying health concerns.

So most people who have had H1N1 have not been confirmed by a lab. Based on assessments earlier in the year in Australia and the United States, for every one lab-confirmed case of H1N1, there are another 135 people who have the virus.

That means thousands of cases of suspected H1N1 could be something else entirely. Even when the second wave of the virus was peaking a month ago, only half of the cases tested positive when examined in a lab.

"Most people will not have had laboratory testing on their respiratory illness," noted Travis Hottes, surveillance epidemiologist for the B.C. Centre for Disease Control. "There is no way, just based on the symptoms, to differentiate between influenza or one of these respiratory illnesses."

Which means people who think they don't need to be vaccinated because they believe they have already been exposed are gambling against poor odds, he said. "It's a guessing game and there isn't a good solid way to find out. We're recommending people get the vaccine unless they have had laboratory-confirmed evidence of this pandemic influenza virus."

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In B.C., the Centre for Disease Control tested nearly 2,500 specimens a month ago when the virus was peaking. As in Ontario, roughly half of the swabs tested positive for influenza, and virtually all of the flu cases have proved to be H1N1.

Every week since, the rate of H1N1 in lab tests has dropped. In B.C. last week, 34.7 per cent of the 1003 samples tested positive. In Ontario, just 22.8 per cent of the specimens tested positive last week.

"I think people, despite all the messaging, are still confused about what the flu actually looks like," Dr. Gardam said. "The flu is really muscle aches and headaches, usually with a cough, usually with a fever, and you just feel miserable."

Because there are a number of common viruses that mimic those symptoms, doctors and patients alike can be fooled.

B.C.'s chief medical health officer, Perry Kendall, noted there are still a significant number of cases of H1N1 around and there is still a potential threat of another wave hitting Canada in the New Year.

"Because we are on the downside of the curve, there is less H1N1 around than there was, but there is still a considerable amount of H1N1 around and it's going to be around through the winter, I suspect," Dr. Kendall said.

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"The one thing you can confidently predict, though, is that if you had sufficient percentage of the population protected, you won't see a third wave."

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