# 7426
The only real constant with influenza strains is that they are constantly changing. As viruses, they leave behind (varying degrees) of immunity in every host they infect. Were they not to change, they would eventually run out of susceptible hosts.
Influenza viruses evolve via two well established routes; Antigenic drift & Antigenic Shift (reassortment).
Antigenic drift causes small, incremental changes in the virus over time. Drift is the standard evolutionary process of influenza viruses, and often come about due to replication errors that are common with single-strand RNA viruses.
Shift occurs when one virus swap out chunks of their genetic code with gene segments from another virus. This is known as reassortment. While far less common than drift, shift can produce abrupt, dramatic, and sometimes pandemic inducing changes to the virus.
And while we talk about the four main strains of influenza that are currently circulating in humans (A/H1N1(pdm), A/H3N2, B Victoria, B Yamagata) as if they were single entities - in reality – within each strain, you will find a good deal of diversity.
New `prototypes’ from these strains are constantly being generated (mostly by antigenic drift) and `field tested’ for biological fitness and transmissibility.
Most are evolutionary failures.
But occasionally, a new, biologically fit virus will emerge that can compete with its parental strains, and it begins to spread.
NIAID has a terrific 3-minute video that shows how influenza viruses drift over time, and why the flu shot must be frequently updated, which you can view at this link.
As flu vaccine formulations must be decided upon six months in advance of each flu season, public health agencies like the CDC, ECDC, the World Health Organization, Hong Kong’s CHP (and others) spend considerable resources on influenza surveillance, looking for signs of any up-and-coming viral strains.
All of which brings us to the ECDC’s latest influenza virus characterization summary, that looks at the ongoing evolution of these seasonal strains over the past 6 months.
Influenza virus characterisation: Summary Europe, December 2012 to May 2013
26 Jun 2013
ECDC
The latest issue of ECDC’s monthly series on 'Influenza virus characterisation’ covers the time period from 1 December 2012 to 31 May 2013, spanning the entire 2012-13 season.
It is prepared by the European Reference Laboratory Network for Human Influenza (ERLI-Net). Until June 2013, ERLI-Net was called the Community Network of Reference Laboratories for Human Influenza in Europe (CNRL).
During the 2012–13 season, A(H1N1)pdm09, A(H3N2) and B/Victoria- and B/Yamagata-lineage influenza viruses have been detected in ECDC-affiliated countries. The relative prevalences varied between countries.
The report summarises the findings as follows:
- Type A and type B viruses have continued to co-circulate in similar proportions.
- A(H1N1)pdm09 viruses have been detected at comparable levels to A(H3N2) viruses.
- A(H1N1)pdm09 viruses continued to show genetic drift from the vaccine virus, A/California/07/2009, but the vast majority remained antigenically similar to it.
- The vast majority of A(H3N2) viruses have been antigenically and genetically similar to cell-propagated A/Victoria/361/2011, the prototype vaccine virus for the 2012–13 influenza season.
- Viruses of the B/Yamagata lineage predominated over those of the B/Victoria lineage.
- B/Victoria lineage viruses were antigenically similar to cell-propagated reference viruses of the B/Brisbane/60/2008 genetic clade.
- Recent B/Yamagata-lineage viruses fell into two antigenically distinguishable genetic clades: clade 2, represented by B/Estonia/55669/2012, and clade 3, represented by B/Wisconsin/1/2010 (the recommended vaccine component for the 2012–13 influenza season).
For further details, download the complete report 'Influenza virus characterisation - Summary Europe, May 2013'.
Last February the World Health Organization met with influenza expert from around the globe to decide on this fall’s flu vaccine composition. Their decision:
Recommended composition of influenza virus vaccines for use in the 2013-14 northern hemisphere influenza season
21 February 2013
It is recommended that trivalent vaccines for use in the 2013-14 influenza season (northern hemisphere winter) contain the following:
- an A/California/7/2009 (H1N1)pdm09-like virusa;
- an A(H3N2) virus antigenically like the cell-propagated prototype virus A/Victoria/361/2011b*;
- a B/Massachusetts/2/2012-like virus.
It is recommended that quadrivalent vaccines containing two influenza B viruses contain the above three viruses and a B/Brisbane/60/2008-like virusc.
a A/Christchurch/16/2010 is an A/California/7/2009-like virus;
b A/Texas/50/2012 is an A(H3N2) virus antigenically like the cell-propagated prototype virus A/Victoria/361/2011;
c B/Brisbane/33/2008 is a B/Brisbane/60/2008-like virus.
* It is recommended that A/Texas/50/2012 is used as the A(H3N2) vaccine component because of antigenic changes in earlier A/Victoria/361/2011-like vaccine viruses (such as IVR-165) resulting from adaptation to propagation in eggs.
It is always difficult – six months in advance – to predict which flu strains are likely to predominate in the upcoming flu season. Some years the vaccine is a good match, other years, not so much.
The good news is that despite the inevitable evolution of the flu strains in circulation, so far, the vast majority of those tested by the ECDC are described as being antigenically similar to the components of this year’s flu vaccine.
