# 5237
Like the UK and most of Europe, Hong Kong is reporting a strong return of the 2009 H1N1 `swine’ flu this year, in stark contrast to the H3N2 dominated flu season being experienced here in the US and Canada.
In fact, when you look at the latest global flu surveillance figures from the World Health Organization, you’ll find major differences in the predominate viruses currently circulating across Asia, Australia, and Europe.
- In Russia, it is primarily influenza B.
- In China, it’s H1N1, followed by H3N2, and then influenza B.
- In western Europe, mostly H1N1, followed by influenza B, and with very low levels of H3N2
Today, Hong Kong’s CHP (Centre for Health Protection) issued a statement on their recent spike in influenza cases, attributing roughly 3/4ths of all influenza cases to the H1N1 virus.
Hong Kong enters influenza peak season
The Centre for Health Protection (CHP) of the Department of Health today (January 17) called on the public to maintain vigilance against seasonal influenza as latest surveillance data showed that Hong Kong has entered the winter influenza peak season.
The Public Health Laboratory Centre (PHLC) of the CHP has detected increases in the number of influenza isolations recently. A total of 149 influenza detections were recorded last week (ending January 15, 2011) compared with 26, 47 and 77 in the weeks ending December 25, 2010, January 1 and January 8, 2011 respectively. The number of influenza-like illness outbreaks occurring in institutions and schools also increased from three in the week ending January 8 to 17 in the week ending January 15.
The majority (76%) of influenza viruses isolated in the recent two weeks were influenza A (H1N1) 2009 (i.e. human swine influenza), followed by influenza A (H3N2) (12%) and influenza B (12%).
The number of other respiratory viral infections detected by PHLC, including Respiratory Syncytial Virus (RSV), parainfluenza virus and adenovirus, have also increased.
For the first three-quarters of the 20th century, the world apparently only had to contend with one influenza A strain at a time. In fact, that was believed to be the normal scheme of things.
One A strain ruled until a new strain emerged to supplant it.
We assume that’s what happened in 1918 with the H1N1 `Spanish flu’ (`assume’ because we lack the ability to determine what viruses circulated prior to 1918), in 1957 when the H2N2 `Asian’ flu replaced H1N1, and again in 1968 with the H3N2 `Hong Kong’ flu replacing H2N2.
It wasn’t until 1976, when the H1N1 virus suddenly reappeared after an absence of 20 years (sparking an epidemic mostly in children), that we saw two influenza A viruses co-exist each year, bumping shoulders, jostling for position, and playing a viral version of king-of-the-mountain.
Since then, some years H3N2 dominated, and some years it was the H1N1 virus. We’ve had two B strains (Yamagata & Victoria) co-circulating since the 1980s as well.
When the novel H1N1 virus emerged in 2009, it seemed to have the potential to supplant both of these previously circulating influenza A viruses.
And for a time, it appeared that it had. Reports of the old `seasonal H1N1’ and H3N2 viruses had all but vanished a year ago.
Starting last spring, however, we began to see an increase in H3N2 cases globally, and this fall, in the Western Hemisphere, it has been the dominant strain.
While exceedingly rare, we occasionally still see an old seasonal H1N1 virus as well, as was recently reported in Canada by GISN. There obviously remains some small reservoir of hosts keeping that strain on life support.
Midway through our convoluted flu season here in the Northern Hemisphere, one can’t help but wonder what surprises lay ahead when the flu moves south of the equator in a few more months.
Stay tuned. It’s bound to be interesting.
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