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Up until 1977 it was considered normal that only one strain of influenza A circulated at a time.
When a new pandemic strain appeared (as it did in 1918, 1957, and 1968), the existing seasonal strain – for reasons that weren’t well understood – would vanish, and in time the new pandemic virus would settle in as the seasonal flu.
But in 1977 something unprecedented happened.
The H1N1 flu virus – which had been replaced by the pandemic H2N2 virus in 1957 (only to be replaced by the H3N2 strain 11 years later) - suddenly reappeared after 20 years absence.
The theory is that it escaped from a Russian or Chinese lab’s freezer, as it was almost unchanged from a strain not seen since the early 1950s.
It was dubbed the `Russian flu’, and for a while, slammed Emergency rooms and Flu wards in a big way.
Most adults had some immunity left over from their exposures before 1957, and so kids under 20 were the hardest hit. But unlike in 1918, 1957, and 1968 – this new flu didn’t depose the old flu.
Instead we ended up with two major Influenza A strains bumping shoulders and jockeying for position. Some years H3N2 would dominate, and other years it would be H1N1.
Further complicating matters we also have Influenza B viruses, which while generally regarded as less serious than influenza A, have divided into two distinct lineages (Yamagata & Victoria.
And they too compete each year for dominance.
Which leaves us with (currently) four major flu strains to contend with;
These viruses constantly change and evolve over time, and we often have several clades of each strain at any given time. The most recent ECDC: Influenza Virus Characterization found 5 genetically distinct H3N2 strains in circulation across Europe.
The recent emergence of a new – antigenically different – H3N2 strain, along with the recent dominance of the Yamagata influenza B virus, has prompted a major change in this year’s flu vaccine.
- The H1N1 component remains essentially unchanged, with the A/California/7/2009 (H1N1)pdm09-like still recommended.
- But the old A/Perth/16/2009 (H3N2)-like virus now gives way to the A/Victoria/361/2011 (H3N2)-like virus.
- And the Victoria lineage B/Brisbane/60/2008-like virus will be replaced by a Yamagata strain; the B/Wisconsin/1/2010-like virus.
Which makes getting the flu vaccine this fall doubly important, as it is likely that community immunity to both of these strains is low.
Beyond these four, keeping researchers up late at night is the fact that there are no fewer than six influenza viruses that – while not well adapted to humans – keep trying to nudge their way into the human host pool.
Most people by now are aware of the concerns over the H5N1 `bird flu’, but less well known are the H7 avian strains, which have managed to jump to humans several times over the past decade.
- In 2003 an outbreak of H7N7 at a poultry farm in the Netherlands went on to infect at least 89 people. Most of the victims were only mildly affected, but one person died.
- In 2004 two people in British Columbia tested positive for H7N3 (see Health Canada Report) during an outbreak that resulted in the culling of 19 million birds.
- In 2006 and 2007 there were a small number of human infections in Great Britain caused by H7N3 (n=1) and H7N2 (n=4), again producing mild symptoms.
H9N2 is another avian strain that has, on rare occasions, infected humans and is believed to have some pandemic potential. Over the past dozen years a small handful of cases have been identified – mostly in Hong Kong (see CIDRAP Avian Influenza (Bird Flu): Implications for Human Disease).
- In January of 2010, in H9N2: The Other Bird Flu Threat, I wrote about the World Health Organization recommending the creation of a candidate vaccine virus for H9N2.
- And in November 2010, in Study: The Continuing Evolution Of Avian H9N2, we looked at computer modeling that suggested that the H9N2 virus moving towards becoming more `human-adapted’.
And since 2005 the CDC has been reporting a growing number of swine flu variants that have managed – on rare occasions – to jump to humans.
The three main flu strains circulating in pigs are:
When one of these swine viruses jumps to a human host, it is then called a `variant’ virus.
Up until last year, it was the trH1N1 swine virus (now called H1N1v) that had been most commonly reported. The numbers were very low – rarely more than 2 or 3 infections each year.
Over the past year, the focus has shifted to the H3N2v virus, which emerged in the summer of 2011. After a quiet winter and spring, this summer it has infected several hundred people in the Midwest – most (but not all) appear to have contracted it directly from exposure to pigs.
And as an added surprise, on Friday of last week we learned of 1 confirmed and 2 suspected cases of a variant flu strain that had only been reported twice during the previous 7 years, in Minnesota Reports Swine H1N2v Flu.
Next week this chart will likely need updating.
There are other flu strains out there, of course.
Earlier this year, concerns were raised over an H3N8 flu virus that had killed seals off the coast of New England, and which conceivably could infect other mammals as well (see mBio: A Mammalian Adapted H3N8 In Seals).
And given the propensity for flu viruses to reassort (swap gene segments), the possibility of seeing an entirely new flu strain emerge can’t be ignored.
Nature’s laboratory is open 24/7, and unlike human researchers suffers neither from bureaucratic rules or budgetary constraints.
That said, history has shown that pandemic events only happen rarely; just three times during the last century (albeit with a couple of close-but-no-cigar events thrown in to keep us on our toes).
Even with all of these potential threats on the viral horizon we could easily see average flu season.
So far, none of these novel viruses has demonstrated the ability to transmit among humans in a sustained and efficient manner while producing significant illness.
And if we get lucky, they never will.
But as schools open, and cooler weather arrives, the conditions conducive for the spread of influenza improve. And that holds true whether we are talking seasonal flu, or one of these upstart strains.
Today is day three of National Preparedness Month, and while most people think of emergency preparedness as something you do for hurricanes and earthquakes, flu epidemics are also worth planning for as well.
Preparedness should include practicing good flu hygiene (hand washing-sanitizing, covering cough & sneezes, staying home if sick) all year round, and getting the flu shot early each year.
No one can predict with certainty the kind of flu year we will see, and I certainly have no special insight into the matter. The CDC states it pretty succinctly:
Flu seasons are unpredictable. The CDC and WHO closely monitor flu cases to identify new viruses or potential epidemics or pandemics.
The CDC and the WHO obviously take pandemics and epidemics very seriously, which is reason enough for us to take a more in-depth look at pandemic preparedness later this month as National Preparedness Month continues.