Friday, June 26, 2009

Victoria: H1N1 Crowding Out Seasonal Flu?



# 3397




Flu Timeline


For much of the last century, mankind only had to deal with one circulating influenza `A’ strain at a time.  


When a new pandemic strain showed up, as it did in 1918, 1957, and 1968 – it overran, and replaced, the previously circulating influenza strain. 


Whatever happened before 1918 is a matter of conjecture, as modern testing methods did not exist back then. 


And while scientists weren’t sure exactly why this happened, they came to expect it when a new pandemic virus arrived on the scene.  


It would, in short order, replace the existing flu strain.


Then in 1977 the H1N1 virus, driven into extinction in 1957 by the H2N2 virus, showed up again after a 20 year absence (the H2N2 virus, in turn, had been replaced by the H3N2 virus in 1968).


How H1N1 reemerged is a bit of a mystery, although it seemed to start in Russia (hence it was called the `Russian Flu’), and many people suspect it was the result of some kind of laboratory accident.


Those people who were around in the mid to late 1950s had exposure to the H1N1 virus, and had some acquired immunity.  Those under the age of 20 were the hardest hit.   While not a true pandemic, 1977 will be remembered as a very difficult flu season.


This time, however, the H1N1 virus didn’t replace the H3N2 virus . . . it began to co-circulate along with it. For the first time in our (admittedly) limited experience, we now had two influenza `A’ viruses at the same time.  


The fact that Russian Flu wasn’t a true pandemic across all age groups may explain why this happened. 


Since older people had acquired some immunity to the H1N1 virus in the 1950s, it wasn’t strong enough to drive the H3N2 virus out of that population. 


Vaccine manufacturers suddenly had to move to a trivalent (3 strain vaccine) in order to cover both `A’ strains, and influenza `B’.


In 2002, a third `A’ Strain was identified, the H1N2 – clearly an assortment between the H3N2 and the H1N1 virus.   It appears no more virulent that its parents, and has produced no detectable surge in influenza cases. 


Fortunately, since H1N2 shares the hemagglutinin and neuraminidase surface proteins of the H1N1 and H3N2 viruses, the vaccines for those appear to work for this reassortment.


It is into this crowded field that the novel H1N1`swine’ flu virus enters, and how it interacts with these other viruses is a big unknown.  


Will it co-circulate with the other H1N1, H3N2, and H1N2 viruses?

Or will it replace them, and become the dominant flu strain around the world?


As the H1N1 virus spreads globally, particularly in the Southern Hemisphere where it is there regular flu season, we will watch carefully to see how it acts, and whether it begins to replace the existing flu strains.


According to this report by Jason Gale of Bloomberg news, in the early going in Victoria, Australia, the novel H1N1 virus appears to be driving the other viruses out.  


Swine Flu Supplanting Seasonal Strains in Australia’s Victoria

By Jason Gale

June 26 (Bloomberg) -- Swine flu is supplanting seasonal strains in Australia’s Victoria state, suggesting the pandemic virus will be the major cause of influenza this winter.


Tests on 138 type-A flu samples collected by a network of doctors confirmed 60 cases of the new A/H1N1 variant and only five of seasonal influenza in the eight weeks ended June 21, according to a report today by the Victorian Infectious Diseases Reference Laboratory in Melbourne.


A typical feature of previous pandemics is for a novel virus to crowd out the most common flu strains during winter, Margaret Chan, director-general of the World Health Organization, said this month. It’s also a factor the agency will consider in determining when vaccine makers should switch to producing only shots against the pandemic virus, which is spurring an increase in flu cases in Australia including New South Wales state.


“We are seeing a big increase compared to the same stage of the flu season last year, and the increase is predominantly in 5- to 16-year-olds,” Kerry Chant, chief health officer for New South Wales, said in a statement today. Two children are in intensive care and two other people hospitalized, she said.


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