Will we have to add another Influenza A strain to this chart?
# 6034
Although our knowledge of what influenza viruses were in circulation prior to 1918 is very limited, we do know that from 1918 to 1977 only one influenza A virus seemed to circulate in humans at a time.
In 1918, the H1N1 virus appeared (although it wasn’t isolated until the 1930s), replacing whatever had been in circulation before it. And H1N1 reigned supreme for nearly 40 years, until a new upstart virus – called H2N2 – appeared with the pandemic of 1957, driving H1N1 into obscurity.
Eleven years later, another pandemic virus – called H3N2 – arrived with the Hong Kong Flu of 1968, and H2N2 followed H1N1 into the viral dustbin.
So naturally scientists assumed that the natural order of things was to have only one influenza A virus dominant at a time. True, there might be small isolated outbreaks of differing strains, but it appeared that only one could be king of the viral mountain.
Then in 1977, the H1N1 virus mysteriously reappeared after an absence of 20 years (possibly escaping from a Russian or Chinese research lab) . . . and this time, it did not drive out the currently circulating H3N2 virus.
Since those over the age of 20 had some immunity to the H1N1 virus, it mainly affected children. Some scientists believed that this left a substantial reservoir of adults susceptible to the H3N2 virus, allowing its survival.
Suddenly we had two influenza A viruses (H1N1 & H3N2) co-circulating - along with a couple of B strains - and that status quo continued until the spring of 2009 when a new H1N1 swine flu virus arrived on the scene.
While the new H1N1 virus supplanted the old seasonal H1N1 virus, it did not displace the H3N2 virus, which again left us with two co-circulating influenza A viruses.
Fast forward a couple of years, and today we are seeing some new variants of both H3N2 and H1N1 swine viruses popping up, apparently the result of the 2009 pandemic virus re-entering the swine population and reassorting with other swine flu viruses to produce hybrids.
While the number of human infections with these novel flu viruses remains small, the concern is that one (or perhaps more) of these variant viruses could end up adapting well enough to humans to begin to spread efficiently.
Most researchers are downplaying the pandemic potential for these variant viruses – at least in their present form – since there is probably a fair amount of community immunity to them, particularly among adults.
While the future of these variant viruses is impossible to see, the possibility exists that we could end up seeing three influenza A viruses co-circulating.
Helen Branswell of the Canadian Press brings us more on this possibility in her report from last night. As always with a Branswell story, you should follow the link and read it in its entirety.
U.S. finds new human infection with swine H3N2 flu
Updated: Fri Dec. 23 2011 17:16:22
The Canadian PressU.S. public health officials have found another case of human infection with a swine-origin H3N2 virus, this time in a child from West Virginia. And they also reported finding a human infection with a new swine influenza virus never before seen in humans, in a person in Wisconsin who had contact with pigs.
Yesterday the CDC prepared and released a small avalanche of guidance documents on these emerging variant viruses.
Yesterday I linked to Guidance For People Involved In Raising Swine and the early release MMWR on H3N2v Transmission and Guidelines.
Today, you’ll find links to interim guidance documents designed for Healthcare professionals.
- Interim Guidance on Case Definitions to be Used For Investigations of Influenza A(H3N2)v Virus Cases
The public health risk from these variant viruses appears low at this time, but the CDC reminds us:
While there is no evidence that sustained human to human transmission is occurring, all influenza viruses have the capacity to change and it's possible that this virus may become widespread.
Which makes maintaining good flu hygiene this winter (washing/sanitizing your hands, covering coughs & sneezes, staying home when sick), and getting your seasonal flu shot, your best strategies to avoid getting sick during holiday season.
