# 6520
While we talk about four main strains of influenza that currently circulate in humans (A/H1N1(pdm), A/H3N2, B Victoria, B Yamagata), in reality – within each strain - you will find a good deal of diversity.
Influenza viruses are notoriously unstable, mutating at a rapid pace in order to evade acquired immunity. So it isn’t unusual to find numerous genetic variations within the same strain.
Even within a single infected host, you’ll find mutations occurring as the virus replicates. Most go nowhere, being genetically flawed, or unable to compete with its more `biologically fit’ parental viruses.
But occasionally a competitive strain emerges and crowds out the others. So over time, multiple variations of a viral theme can end up circulating in the human population simultaneously.
Sometimes these mutations can move the virus far enough away from earlier strains so that existing host defenses are no longer able to recognize it.
That reduces (or eliminates) the immunity acquired by previous exposure or vaccination.
When a strain is said to be `antigenically similar’ to the vaccine strain, it is expected (but not assured) that the vaccine remains reasonably effective.
But as these genetic changes accumulate, the effectiveness of a vaccine may eventually erode, and the vaccine strains must be replaced.
All of this is part of the normal evolution of influenza viruses, known as antigenic drift. It explains why it is necessary to re-evaluate, and change, the flu vaccine every year or two.
NIAID, which is part of the NIH, has a short (3 minute) video that nicely illustrates how flu viruses change antigenically over time, and can eventually mutate so that the current flu vaccine no long is effective.
Which brings us to the latest ECDC influenza virus characterization report for Europe.
As you will see, there is a growing diversity in both the H1N1(pdm) and H3N2 virus strains, and they take note of the emerging H3N2v virus in the United States.
First the abstract, and a link to the report:
Since 1 January 2012, influenza A(H1N1)pdm09, influenza A(H3N2) and influenza B/Victoria and B/Yamagata lineage viruses have been detected in ECDC-affiliated countries.
- Type A viruses have predominated over type B.
- A(H3N2) viruses have predominated over A(H1N1)pdm09 viruses.
- A(H1N1)pdm09 viruses continue to show genetic drift from the vaccine virus, A/California/07/2009, but the vast majority remain antigenically similar to it.
- During this time period, all European A(H3N2) viruses sequenced fell within five genetic groups. Test viruses isolated in mammalian cells show low titres with post-infection ferret antisera raised against egg-propagated viruses, including the new vaccine virus A/Victoria/361/2011. They react well with post-infection ferret antisera raised against A/Victoria/361/2011 and other current reference viruses propagated exclusively in tissue culture.
- Recent B/Victoria lineage viruses fell within the B/Brisbane/60/2008 genetic clade and were antigenically similar to reference cell-propagated viruses of the B/Brisbane/60/2008 genetic clade.
- Recent B/Yamagata-lineage viruses fell into two genetic clades, represented by B/Bangladesh/3333/2007 and B/Wisconsin/1/2010 (Clade 3) or B/Brisbane/3/2007 (Clade 2); viruses in these clades are antigenically distinguishable.
- Antigenic analysis of A(H3N2)v viruses, the cause of zoonotic infections in the USA, indicate that they are antigenically distinct from seasonal A(H3N2) viruses.
While the technical details of this report will be of most interest to virologists, epidemiologists, and vaccine manufacturers . . . the bottom line is that influenza viruses are a constantly moving target, and as they mutate and change, their ability to infect us improves.
Which means that even if you didn’t get the flu last year or the year before, that doesn’t guarantee you won’t get it this year or next.
While certainly not 100% protective against all of the flu strains out there, this fall’s reformulated flu shot (see Northern Hemisphere 2012-2013 Flu Vaccine Composition) - along with practicing good flu hygiene - remain your best protections against catching the flu this winter.
