Snowy Owl
Retired in 2010, In Memoriam
Variation between Canadian, U.S. flu seasons persists; that's flu, experts say
http://canadianpress.google.com/article/ALeqM5iMqtxn2y1c6IfTJ4Jrs-tASePLMA
http://canadianpress.google.com/article/ALeqM5iMqtxn2y1c6IfTJ4Jrs-tASePLMA
TORONTO
One of the mantras of global public health is that infectious diseases know no borders.
But this year's flu season continues to thumb its nose at that notion, rolling out in distinctly different patterns on either side of the 49th parallel.
Maps generated by the Public Health Agency of Canada and the U.S. Centers for Disease Control show widespread flu activity in 49 U.S. states, but in only one province, British Columbia.
Other parts of Canada are experiencing localized or even sporadic outbreaks; some may be past the peak of their flu seasons.
What explains the differences? Experts shake their heads and marvel at the enduring ability of the famously unpredictable influenza virus to confound explanation and prediction.
Ask Dr. Nancy Cox, head of the CDC's Influenza Division, if she's surprised to see distinctly different seasons playing out on the upper and lower part of the continent, and her response reflects her many years of study of this virus.
"Yes and no," says Cox, who is credited with having coined the oft-repeated expression, "If you've seen one flu season, you've seen one flu season."
"It surprises me because influenza viruses really do not respect boundaries.
It doesn't surprise me because we've seen these very striking differences between influenza season in the U.S. and the influenza season in Canada in the past - and haven't been able to explain the differences."
For ease of discussion, people talk about influenza as if it were an entity.
But in fact, there are three types that cause human disease - influenza A, B and to a lesser degree C. There are two subtypes of A viruses, H3N2 and H1N1. And because the viruses mutate constantly, there are families of viruses within each of those types.
In Canada this year, two families of H3N2 viruses, one H1N1 virus, and two influenza B virus groups are circulating and causing disease. The flu shot gives people who get it protection against one H1N1, one H3N2 and one B virus.
But the main viruses causing disease so far this year in Canada are H1N1s that are closely matched to the one included in this year's flu shot.
So people who got a flu shot - and as many as about a third of Canadians do - would be well positioned to ward off assault from those viruses.
In the United States, a recent surge in H3N2 activity has made those viruses the most predominant cause of illness there this year. The H3N2 viruses circulating aren't well matched to the vaccine.
The latest data from the Public Health Agency of Canada doesn't yet show a similar H3N2 surge here, though Dr. Danuta Skowronski of the British Columbia Centre for Disease Control wouldn't rule it out yet.
Skowronski says British Columbia is seeing an uptick in H3N2 cases and suggests other areas might as well.
"We are starting to detect more H3 (cases). Now whether H3 will be able to assert itself before the typical spring climate changes that are not very favourable towards the influenza virus, or whether it's going to be sort of kept at bay here . . . I don't know," she says.
Skowronski does have a theory about why H3N2 viruses are less common in Canada this winter, though, saying a study she and colleagues did looking at gene sequences of flu viruses from last year showed the newest family of H3 viruses, known as Brisbane 10-like viruses, started to swing through Canada last winter.
"So it may be that there is some pre-existing immunity that's attenuating the impact of that H3N2 here," she says.
Scientists who work in the field of influenza expect the unexpected - though it can still surprise them when they see the virus's latest curve ball.
"It is just so humbling to be shown how often this virus just does things that you don't expect," says Dr. Keiji Fukuda, co-ordinator of the World Health Organization's Global Influenza Program.
"We are always surprised enough to be humbled by it."
One of the mantras of global public health is that infectious diseases know no borders.
But this year's flu season continues to thumb its nose at that notion, rolling out in distinctly different patterns on either side of the 49th parallel.
Maps generated by the Public Health Agency of Canada and the U.S. Centers for Disease Control show widespread flu activity in 49 U.S. states, but in only one province, British Columbia.
Other parts of Canada are experiencing localized or even sporadic outbreaks; some may be past the peak of their flu seasons.
What explains the differences? Experts shake their heads and marvel at the enduring ability of the famously unpredictable influenza virus to confound explanation and prediction.
Ask Dr. Nancy Cox, head of the CDC's Influenza Division, if she's surprised to see distinctly different seasons playing out on the upper and lower part of the continent, and her response reflects her many years of study of this virus.
"Yes and no," says Cox, who is credited with having coined the oft-repeated expression, "If you've seen one flu season, you've seen one flu season."
"It surprises me because influenza viruses really do not respect boundaries.
It doesn't surprise me because we've seen these very striking differences between influenza season in the U.S. and the influenza season in Canada in the past - and haven't been able to explain the differences."
For ease of discussion, people talk about influenza as if it were an entity.
But in fact, there are three types that cause human disease - influenza A, B and to a lesser degree C. There are two subtypes of A viruses, H3N2 and H1N1. And because the viruses mutate constantly, there are families of viruses within each of those types.
In Canada this year, two families of H3N2 viruses, one H1N1 virus, and two influenza B virus groups are circulating and causing disease. The flu shot gives people who get it protection against one H1N1, one H3N2 and one B virus.
But the main viruses causing disease so far this year in Canada are H1N1s that are closely matched to the one included in this year's flu shot.
So people who got a flu shot - and as many as about a third of Canadians do - would be well positioned to ward off assault from those viruses.
In the United States, a recent surge in H3N2 activity has made those viruses the most predominant cause of illness there this year. The H3N2 viruses circulating aren't well matched to the vaccine.
The latest data from the Public Health Agency of Canada doesn't yet show a similar H3N2 surge here, though Dr. Danuta Skowronski of the British Columbia Centre for Disease Control wouldn't rule it out yet.
Skowronski says British Columbia is seeing an uptick in H3N2 cases and suggests other areas might as well.
"We are starting to detect more H3 (cases). Now whether H3 will be able to assert itself before the typical spring climate changes that are not very favourable towards the influenza virus, or whether it's going to be sort of kept at bay here . . . I don't know," she says.
Skowronski does have a theory about why H3N2 viruses are less common in Canada this winter, though, saying a study she and colleagues did looking at gene sequences of flu viruses from last year showed the newest family of H3 viruses, known as Brisbane 10-like viruses, started to swing through Canada last winter.
"So it may be that there is some pre-existing immunity that's attenuating the impact of that H3N2 here," she says.
Scientists who work in the field of influenza expect the unexpected - though it can still surprise them when they see the virus's latest curve ball.
"It is just so humbling to be shown how often this virus just does things that you don't expect," says Dr. Keiji Fukuda, co-ordinator of the World Health Organization's Global Influenza Program.
"We are always surprised enough to be humbled by it."