03/06/2012 06:50 EST | Updated 05/06/2012 05:12 EDT

Northern Hemisphere Flu Season: Why So Few Cases?

TORONTO - It's a bit confounding, even for influenza.

This year's Northern Hemisphere flu season has been markedly mild in many places — Canada, the United States, much of Europe.

With the clock ticking down towards what would normally be the end of flu season, people are asking why this year has seen so little influenza activity.

It's an interesting question. But if you're looking for a definitive answer, you may need to wait a few decades until science figures out more about this highly unpredictable virus.

"There's so much we don't know about flu. And one of the biggest things we don't know is why it's bad some years and why it's not bad others," says Dr. Anthony Mounts, a flu specialist with the World Health Organization in Geneva.

Theories abound in the mainstream media. Maybe this year's mild winter has knocked out flu? Maybe people aren't clustering indoors as much, because the temperatures haven't been so harsh? Maybe with increased use of flu vaccine, the virus has fewer people to infect?

In reality, flu experts say there is unlikely to be one single answer when it comes to explaining flu's virtual no-show this year.

"There's lots of evidence that it's not as simple as temperature," says Dr. Allison McGeer, a flu expert at Toronto's Mount Sinai Hospital.

"People have been looking for temperature and humidity indications for a long time — and there may well be some contribution — but if it is, it's subtle and complex. It's very clear that it's not just having a milder than usual winter that makes a difference."

Besides, the mild temperatures argument starts to fall apart when you look further afield. Parts of Europe have had a pretty tough winter, and yet flu activity there is also quite limited, says Mounts.

And Dr. Lone Simonsen, a flu epidemiologist at George Washington University in Washington, D.C., has just published a scientific paper on the difficult fourth wave of H1N1 activity gripping Mexico this winter, an outbreak caused by the virus that triggered the 2009 pandemic.

"It's pretty hot in Mexico," Simonsen says with a laugh. "I'm sure it's a mild winter."

Likewise, experts don't believe increased use of flu vaccine can explain the low levels of activity. For one thing, vaccination rates in Canada and the United States — among the heaviest flu vaccine users in the world — aren't that high. Vaccinating between 35 and 40 per cent of a population isn't enough to stifle influenza, experts say.

And again, the international situation is instructive. Flu vaccine use is much lower in Europe than it is in North America, and Europe's activity is not substantially greater than North America's this winter.

"I think with the kind of coverage levels we're seeing we can't attribute the low level influenza activity we've seen this year to that. I think it doesn't hurt ... but I think there are more complex factors at play than that," says Dr. Danuta Skowronski, a flu expert with the B.C. Centre for Disease Control in Vancouver.

It's worth noting that while flu levels have been low, there has been influenza activity this year.

Skowronski says British Columbia's flu season has percolated at low levels for weeks — never hitting the sharp peak normally seen in flu season, but not going away either.

McGeer says Toronto is in the midst of a moderate wave of influenza B activity, with some outbreaks in long-term care facilities and about 250 hospitalizations for flu in the past two weeks. The supply of beds in hospitals around the city is getting tight, she says, a sign at this time of year of an increase in flu activity.

There is one factor flu experts think may be a partial explanation for this year's low level of flu activity. The circulating flu viruses have been around for awhile — as evidenced by the fact that the viruses in the flu shot haven't been updated for a few years.

The committee that advises the World Health Organization on what strains of H3N2, H1N1 and influenza B viruses to put in the Northern Hemisphere flu shot met late last month in Geneva.

They saw enough small changes in the H3N2 viruses circulating to recommend a new strain for next winter's shot. But the H1N1 virus hasn't changed substantially since the 2009 pandemic, and the strain in the vaccine is still the one used when the virus first emerged.

Because the viruses have circulated for some time, a lot of people will have antibodies, either from having been infected or having received a flu shot.

In a related idea, Skowronski says there is growing interest in a hypothesis that being infected with one type of flu may give people short-term protection against other subtypes of the virus.

If true, that might mean a few active seasons in the wake of the 2009 pandemic have left more people with protection than would normally be the case. But this is still a theory.

Another possibility is that flu hasn't yet shown us everything it has in store for this year. "We just don't know at this point," says Dr. Nancy Cox, head of the influenza division at the U.S. Centers for Disease Control.

"It's possible that influenza will just fade away. Or it's possible that (flu) B will really take off. Or even H3 will continue to circulate and cause more disease than it has so far."

With such a mild season, it's tempting to wonder if flu is mustering for a major assault next year. But people who have tracked influenza for years know better than to make those kinds of predictions.

"Flu is something I don't bet on," Mounts says. Adds McGeer: "Nothing we know about flu presages anything. We get what we get next year. That will be life."

"There are all sorts of things in the natural world that we don't have the ability to predict," she says. And the severity of the hurricane season and the severity of the flu season are just two of them."