Hot Trends: H1N1 activity in Canada normal, experts say

Flu season has swung into full gear in many parts of the country, sowing illness, absenteeism and in some places alarm.

Reports of deaths of adults in Texas and Alberta have raised questions of whether something new is afoot. And where most years by now flu shot efforts would be winding down, several provinces are leveraging intense media interest and public concern by mounting extra clinics for those reconsidering getting a shot.

Is this season shaping up to be a dismal flu season? Or do we, as a society, tend to forget what influenza can do from one season to the next?

There are plenty of such questions being posed these days. Here are a few, and some answers. Q: All the headlines these days are about influenza. Is this a dire flu season?

Not really. It's a flu season - which means as activity takes off in your municipality or province, you can expect to see a spike of illnesses that can create the impression that almost everyone is sick.

If you look at a graph of influenza activity over the year, you see that people can contract influenza at any point. But most cases occur in a peak - or two peaks in some seasons.

Last year's season, caused mostly by the H3N2 virus, was quite nasty. This year the H1N1 virus is predominating - at this point, anyway. And while parts of the country are now in the throes of full-on flu outbreaks, the activity level is what one sees during flu season.

"It's looking like a kind of a solid H1N1 season," says Dr. Allison McGeer, head of infection control at Toronto's Mount Sinai Hospital, and one of Canada's leading flu experts.

McGeer suggests Alberta's outbreak may have already peaked while Ontario's is probably at its apex now.

"It'll be over in a month. There's been a fair amount of activity, but it's not terrible. And that should be it." Q: Isn't H1N1 the virus that caused the 2009 pandemic? Is it different or more dangerous than the other flu viruses?

H1N1 is the virus that caused the pandemic in 2009. But it's been circulating for nearly five years, and is now considered one of the seasonal flu viruses. The others are H3N2, also an influenza A virus, and influenza B viruses.

While the vast majority of people who contract flu don't need medical attention, influenza does have the potential to cause serious illness - and that is true of H1N1, H3N2 and influenza B. Q: There have been reports in the media of young and middle aged adults dying. Doesn't flu normally kill old people?

What is different about H1N1 is who is vulnerable to it and who it seems to hit hard. Dr. Danuta Skowronski of the British Columbia Centre for Disease Control has been studying this.

Skowronski - who is also a prominent flu researcher - and some colleagues recently tested blood samples from people across the age spectrum, trying to determine how much vulnerability there is to H1N1 in the population.

Little kids - children born after the pandemic - are most likely to be susceptible to H1N1; fewer than 20 per cent of them had antibodies against it. And older adults, who don't fare very well in H3N2 years, seem as a group to have a lot of protection against H1N1.

But young adults and people in middle age are less likely to have antibodies to these viruses, and those are the serious illnesses and deaths that are getting a lot of attention in the media this year.

Just to be clear: People die every year from flu. But often those deaths are in senior citizens, an age group in which death is not rare. Deaths in younger adults aren't so expected and may be standing out more starkly because of the relative absence of severe illness in older people, Skowronski says. Q: Alberta has reported nearly 1,000 confirmed cases, and Toronto Public Health says it has confirmed 210 cases of flu so far this year. What do these numbers tell us? Is there more flu than usual?

Numbers don't mean much when it comes to seasonal flu and they probably shouldn't concern the public to any real degree, McGeer says.

The reality is that most people who have flu never see a doctor, and many of those who do aren't tested for influenza. (In fact, until relatively recently most weren't tested. But use of flu tests is rising, hence places like Alberta and Toronto have numbers like this to give out.)

The important point here is that for every confirmed case, there are many, many more infections that haven't been detected. That's why flu experts often use the analogy of an iceberg when they talk about flu. The cases you see are a small fraction of the total out there.

If Alberta has 1,000 positive flu tests, it means there's quite a bit of influenza in Alberta. But is that amount abnormal? Not at the peak of an active flu season.

"The value of counting cases when they don't actually represent anything like the total ... we've got to work on that. It's really hard to explain to people," McGeer says. She suggests that as the use of flu tests rise, the figures that would be worth focusing on would be how many flu hospitalizations and deaths are being reported. Q: People in my neighbourhood/town/city are already sick with flu. If I haven't had a flu shot, is there still time to get one? Is it worth my while?

Public health officials will always say "It's never too late to get a flu shot."

And given that flu viruses will continue to circulate - albeit likely at lower levels - for weeks to come, getting a shot now might prevent you from getting sick later.

But a shot won't offer immediate protection. It can take two or three weeks for the vaccine to generate a protective response. So don't be surprised if you get a shot this weekend and get sick next week. The shot won't have made you ill; it just didn't have enough time to do its job.

McGeer says the 2007-08 flu season provides a good argument for going ahead and getting a shot now, even if flu activity appears to be peaking where you are.

That year an H1N1 virus - the old H1N1 virus that was displaced by the pandemic version - caused a peak of illness just after Christmas. Then in late February, H3N2 and influenza B viruses came on strong.

"That would have been a good year to get vaccinated (late)," McGeer says. Q: I remember that in previous years scientists reported on how well the flu shot was working. Do we know how protective this year's shot is against the circulating viruses?

Not yet. Several groups of scientists in Canada, the United States and Europe are gathering data, looking to see how effective this year's vaccine is in preventing influenza. But it will be at least a few weeks before interim estimates are available.