Although we talk about seasonal flu strains like H1N1, H3N2, and influenza B as if they were single entities, in truth within each sub-type are numerous clades, sub-strains, and variants.
These influenza viruses are constantly mutating (via antigenic drift) as they replicate and spread, and are engaged in a perpetual game of viral king-of-the-mountain, as they jostle for dominance and superiority in the global community.
Success for these strains is always fleeting, though, as they leave behind varying degrees of immunity in their hosts and must either evolve or eventually die out for lack of susceptible hosts. All which makes the flu world dynamic and ever-changing, and presents a genuine challenge for vaccine manufacturers to stay ahead of.
To keep abreast of the changes to the flu strains in circulation, labs around the world send samples to the WHO Collaborating Centre in London for classification, and the ECDC publishes reports roughly once a month. These reports can help give us some idea whether the strains contained the latest vaccine match up antigenically with those strains currently circulating.
As you might expect, given the diversity of flu strains in circulation, the best that can be hoped for is that the majority of viruses tested are antigenically similar to the components in the vaccine. That said, we won’t really have a good idea of how well this year’s vaccine will perform until the flu season is over, next spring.
The summary is printed below. The full report is available as a PDF File.
29 Jul 2014
During the 2013–14 season, A(H1N1)pdm09, A(H3N2), B/Victoria- and B/Yamagata-lineage influenza viruses have continued to co-circulate in EU/EEA Member States. The relative prevalence has varied between countries. Viruses with collection dates after 31 December 2013, from 22 countries, have been received by the WHO Collaborating Centre in London.
- Type A and type B viruses have been received at a ratio of nearly 20:1.
- A(H3N2) and A(H1N1)pdm09 viruses have been received in similar numbers.
- Recently circulating A(H1N1)pdm09 viruses belonged to genetic subgroup 6B. Viruses in subgroup 6B are antigenically similar to the vaccine virus, A/California/07/2009.
- Recently circulating A(H3N2) viruses have fallen within genetic group 3C represented by the recommended vaccine virus for the 2013–14 and 2014–15 seasons, A/Texas/50/2012, with viruses of genetic subgroup 3C.3 predominating. Antigenic analysis using antisera raised against cell-propagated H3N2 viruses indicates that the majority of circulating viruses are antigenically similar to those in circulation in the 2012–13 and 2013–14 influenza seasons. Antisera raised against two reference viruses representative of viruses in genetic subgroup 3C.3 – with HA gene sequences encoding several amino acid substitutions compared to other viruses in genetic group 3C.3 – have been prepared. These antisera recognised the majority of test viruses well.
- Two genetic clades of B/Yamagata-lineage viruses continue to circulate: clade 3 represented by B/Wisconsin/1/2010 and clade 2 represented by B/Massachusetts/02/2012 (the recommended vaccine component for the 2013–14 and 2014-15 influenza seasons). Viruses in each clade have been received in similar numbers but with viruses in clade 3 predominating in those samples collected in 2014.
- Antigenic characterisation of two viruses of the B/Victoria lineage was performed in June. Neither virus was recognised well by the antiserum raised against the egg-propagated reference virus, A/Brisbane/60/2008, a virus previously recommended as a component of the trivalent influenza vaccine and recommended as a component of quadrivalent influenza vaccines for 2013–14 and 2014–15 influenza seasons. The test viruses were not recognised well by antisera raised against other reference viruses propagated in eggs. The test viruses were better recognised by some, but not all, antisera raised against reference viruses exclusively propagated in cells. Phylogenetic analysis revealed that all B/Victoria-lineage viruses received in 2014 were in genetic clade 1A, the B/Brisbane/60/2008 genetic clade.