Although we tend to talk of seasonal H1N1, or H3N2 (or the B viruses) as individual entities – in truth within each subtype there are many clades and variants, and they are all evolving over time. The more `biologically fit’ strains tend to succeed, while less fit viruses fade away.
Geographically, these viruses can vary widely, and so the dominate strains in Europe may differ from the dominant strains in North America or Asia.
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. 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 few months. As you will see, there are changes in the evolving fluscape that have necessitated a change to the next year’s Southern Hemisphere Flu shot composition.
Detailed PDF file
During the 2013–14 season, A(H1N1)pdm09, A(H3N2), B/Victoria- and B/Yamagata-lineage influenza viruses co-circulated in EU/EEA Member States. The relative prevalence has varied between countries. The WHO Collaborating Centre in London received viruses with collection dates after 31 January 2014 from 21 EU/EEA countries.
Type A and type B viruses have been received at a ratio of approximately 10:1. A(H3N2) outnumbered A(H1N1)pdm09 viruses at a ratio of 1.4:1.
Recently circulating A(H1N1)pdm09 viruses, in Europe and worldwide, belonged to genetic subgroup 6B. Viruses in this subgroup 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. 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, but those in a newly emerging genetic cluster, 3C.3a, are poorly recognised by some antisera.
Two genetic clades of B/Yamagata-lineage viruses continue to circulate: clade 3 represented by B/Phuket/3073/2013 and clade 2 represented by B/Massachusetts/02/2012 (the recommended vaccine component for the 2013–14 and 2014–15 influenza seasons). Viruses in clade 3 predominate in those samples collected since 31 January 2014.
Only two viruses of the B/Victoria-lineage were antigenically characterised since the July 2014 report. Neither was recognised well by the antiserum raised against the egg-propagated reference virus, A/Brisbane/60/2008, recommended as a component of quadrivalent influenza vaccines for 2013–14 and 2014–15 influenza seasons. Phylogenetic analysis revealed that all B/Victoria-lineage viruses received in 2014 were in genetic clade 1A, the B/Brisbane/60/2008 genetic clade.
In light of the emergence of antigenically distinct groups of influenza A(H3N2) and the altered prevalence of influenza B viruses, the WHO recommended composition of influenza vaccines for use in the 2015 southern hemisphere influenza season differed from that recommended for use in the 2014–15 northern hemisphere influenza season.
Remarkably, the H1N1 strains in circulation remain very stable, and antigenically similar to the vaccine virus which has been unchanged for five years (A/California/07/2009).
The situation with the H3N2 viruses is more variable, with 7 distinct genetic groups detected since 2009, making vaccine strain selection more problematic.
Since the fall 2014 vaccines strains were selected last February, surveillance has detected a rise in the number of antigenically drifted H3N2 isolates, prompting the WHO to recommend a strain change for next year.
The same can be said for recent detections of the Influenza B – Yamagata lineage of viruses, and so the WHO is recommending a change next year for the vaccine as well.
How all of this will affect the performance of this year’s flu vaccine in the Northern Hemisphere is unknown. If we see a heavy H1N1 year, then probably not much. Nor does the detection of these divergent strains in Europe guarantee when, or even if, we’ll see the same strains in North America.
There is always a degree of uncertainty over how good a match the vaccine will be when we enter a new flu season. But even when there is a less than a perfect match – the flu shot can offer significant levels of protection against a virus that claims thousands of American lives every year.
As we’ve discussed before, flu vaccines – while considered very safe – most years can only offer a moderate level of protection against influenza. Their VE (vaccine effectiveness) can vary widely between flu shot recipients, and is often substantially reduced among those older than 65 or with immune problems.
Although the vaccine can’t promise 100% protection, it – along with practicing good flu hygiene (washing hands, covering coughs, & staying home if sick) – remains your best strategy for avoiding the flu (and other viruses) this winter.