Influenza vaccine: selection of strains to be included in the seasonal vaccine for the Northern Hemisphere 2011-2012 – Early indications of seasonal vaccine effectiveness in EuropeArchived

World Health Organization (WHO), Geneva, February 2011Each year in February WHO convenes a meeting with advisers from the nine or so WHO Influenza Collaborating Centres and Essential Reference Laboratories in order to achieve consensus and advise WHO on the optimal formulation of seasonal influenza vaccines for the next Northern Hemisphere Season (2011-12). A similar meeting takes place each autumn making recommendations for the Southern Hemisphere.   This year’s meeting noted the diversity of experience this season so far with North America  and parts of North Asia experiencing  outbreaks of influenza  A(H3N2), and some A(H1N1)2009 and B viruses while Europe experienced only influenza A(H1N1)2009 and B viruses.  The previous seasonal  influenza A(H1N1) was not seen. The meeting noted how the usual tests against ferret antisera indicated that the A(H1N1)2009 viruses remained antigenically homogeneous and closely related to the vaccine virus A/California/7/2009. Sequence analysis of the haemagglutinin (HA) genes of A(H1N1)2009 viruses indicated that they fell into three genetic subgroups but  were antigenically indistinguishable. The A(H3N2) viruses were similarly assessed and the HA genes of recently isolated viruses fell into two phylogenetic clades represented by A/Perth/16/2009 and A/Victoria/208/2009. Phylogenetic  subgroups within both clades have emerged but viruses within these subgroups were antigenically similar to A/Perth/16/2009. For the B viruses the usual problem emerged of both the B/Victoria/2/87 and the B/Yamagata/16/88 lineages circulating with B/Victoria/2/87 lineage viruses continuing to predominate except in China.  The majority of the B/Victoria/2/87 lineage viruses were antigenically closely related to the vaccine virus B/Brisbane/60/2008 though a small number of viruses from several countries were antigenically and genetically distinguishable from B/Brisbane/60/2008-like viruses.

Based on the results of influenza activity, serology tests of the current vaccines and surveillance of the genetic and antigenic characteristics of the circulating viruses, the strains that were recommended for use in influenza vaccine in the Northern Hemisphere for the season 2011-2012: were • A/California/7/2009 (H1N1)-like virus, • A/Perth/16/2009 (H3N2)-like virus • B/Brisbane/60/2008-like virus.

This recommendation for the Northern Hemisphere is the same as for the season 2010-2011 and for the Southern Hemisphere season.

Based on the analyses the meeting concluded that A(H1N1) pandemic 2009, A(H3N2) and B viruses will most likely co-circulate in the Northern Hemisphere 2011-2012 with the likelihood that the pandemic A(H1N1) 2009 viruses will predominate. On the basis of recent epidemiological evidence, it is anticipated that former seasonal A(H1N1) viruses are unlikely to circulate at significant levels during the 2011-2012 Northern Hemisphere season, hence once more it has not been recommended for inclusion in the 2011-2012 influenza vaccine. 

ECDC Comment (25th February 2011):

It is worth noting how tight this timing has to be for this meeting since the regulatory laboratories and vaccine manufacturers have to have a decision early in the year in order that vaccines are ready for the early autumn. Decisions have to be based on the experience of a season that is still in progress.  This emphasises the continuing importance of uninhibited and early sharing of viruses by all countries  from peripheral laboratories, up to National Influenza Centres and then onto the Collaborating Centres. This is one of the prime purposes of WHO Global Influenza  Surveillance Network. (1-3) That mechanism has been at some threat internationally since 2007 with controversies around virus sharing stemming from some countries and its relationship to benefit-sharing though fortunately the system worked well during the 2009 pandemic.(3,4)  However, equally important is the sub-national mechanism in Europe with clinicians continuing to take specimens from patients with suspected influenza, passing them to diagnostic laboratories and then those taking responsibility to pass them onto reference / national laboratories whether the diagnostic laboratory is in the public or private sector.    It needs to be noted that strain selection is one thing but the proof of vaccine effectiveness comes from the field studies. This is impossible to assess formally with trials each season even if they were ethically possible with such a dangerous infection and vaccines of generally proven effectiveness. Instead, observational studies of field vaccine effectiveness are used in Europe. The initial in-season results are available from Spain and the UK indicating that the new seasonal vaccines are effective in preventing laboratory confirmed mild infections of A(H1N1)2009 and B viruses but seemingly not as effective as the adjuvanted vaccines were at this stage following the pandemic.(5,6)  Analyses of the effectiveness against A(H3N2) viruses in North America are not yet available