World Health Organization, Geneva, Switzerland – 25th February 2016, report published here
The World Health Organization (WHO) has agreed on the recommended composition of the trivalent influenza vaccine for the northern hemisphere 2015-2016 influenza season as:
- an A/California/7/2009 (H1N1)pdm09-like virus;
- an A/Hong Kong/4801/2014 (H3N2)-like virus;
- a B/Brisbane/60/2008-like virus (Victoria lineage).
For quadrivalent vaccines containing two influenza B viruses the above three viruses and a B/Phuket/3073/2013-like virus (Yamagata lineage) should be included . Further antigenic and genetic characteristics of recent seasonal influenza viruses are described in the full report of the recommendation . The WHO recommendation is the same as for the southern hemisphere 2016 seasonal influenza vaccine .
Influenza A(H1N1)pdm09 viruses
The A(H1N1)pdm09 viruses circulating from September 2015 to January 2016 in the northern hemisphere were almost all antigenically similar and closely related to the vaccine virus A/California/7/2009. The sequence analysis of the HA genes of A(H1N1)pdm09 viruses indicated that there were two newly emerging sub-clades within the predominating 6B clade (6B.1 and 6B.2). Viruses within sub-clades 6B.1 and 6B.2 are currently not antigenically distinguishable from A/California/7/2009-like viruses .
Influenza A(H3N2) viruses
A(H3N2) viruses circulating fell into the phylogenetic clades 3C.2 and 3C.3. Viruses in sub-clade 3C.2a, represented by A/Hong Kong/4801/2014 (H3N2) reference virus, predominated in all regions of the world .
Influenza B viruses
The B/Yamagata/16/88 and the B/Victoria/2/87 lineage viruses have continued to co-circulate across the world with a predominance of the B/Victoria lineage. The majority of viruses of the B/Victoria were antigenically closely related to the earlier vaccine virus B/Brisbane/60/2008. There was no antigenic drift observed in the B/Victoria –lineage viruses and all viruses fell into genetic clade 1A .
The majority of circulating viruses of the B/Yamagata fell genetically in clade 3. The recently circulating viruses in the EU/EEA are best represented by vaccine viruses that are B/Phuket/3073/2013-like.
All (sub)types of influenza viruses have been circulating in the northern hemisphere with variable levels of activity . With this recommendation, two of the vaccine components, the A(H3N2) and B virus, will be changed in the trivalent vaccines compared to the composition of the northern hemisphere vaccine in 2015-2016 .
The changes in the strain selection are mainly due to the predominance of the circulating viruses in the northern hemisphere as described above. The recommendation is in line with the currently circulating viruses and antigenic characterisation results. As the circulating A(H1N1)pdm09 viruses remain antigenically homogeneous and closely related to the earlier vaccine virus A/California/7/2009 based on the haemagglutinin inhibition test, there is no change in the recommendation for this component. Even so, the early analysis of the 2015-2016 vaccine effectiveness in Europe shows a reduced vaccine effectiveness for the A(H1N1)pdm09 A/California/7/2009 component . The reasons for this reduction in vaccine effectiveness are not understood and have to be interpreted with caution as the early estimates are based on a small number of patients.
The continued antigenic drift in the A(H3N2) viruses, and the difficulties of the egg-grown earlier vaccine component A/Switzerland/9715293/2013 (H3N2)-like virus to induce high titers of antibodies, have been reasons for a change in the A(H3N2) virus vaccine component. During the 2014–2015 season, low vaccine effectiveness was observed, especially against the dominant subtype A(H3N2), due to the mismatch between the vaccine components and the circulating viruses . In New Zealand, during the A(H3N2) predominated 2015 influenza season, vaccine effectiveness was 36% against laboratory-confirmed influenza like illness and 50% against hospitalisations with the same vaccine composition as is suggested for 2016-2017 northern hemisphere season .
For the B viruses, there is an observed change to a predominance of the B/Victoria-lineage. Therefore the rationale for the change in the vaccine composition from Yamagata- to Victoria-lineage virus is clear. For the B/Victoria clade 1A and B/Yamagata clade 3 viruses, sufficient antigenic homology exists with the vaccine components and therefore it is likely that the northern hemisphere vaccine would protect against the circulating viruses. In quadrivalent vaccines, both B lineage components will be present. Based on the higher prevalence of B/Victoria lineage viruses in the southern henmisphere season 2015, it is expected, that the Victoria-lineage prevalence will continue in nothern hemisphere over the 2016-2017 influenza season and therefore even the trivalent vaccine should give a good protection against majority of B viruses.
As both A(H1N1)pdm09 and A(H3N2) viruses continue to drift, it is difficult to estimate with confidence the proportions of circulating viruses next season and the protection by the recommended vaccine components. The technical difficulties in virus culture, characterisation and egg propagation of A(H3N2) viruses, make the selection of new vaccine components more difficult.
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2. World Health Organization. Recommended composition of influenza virus vaccines for use in the 2016 southern hemisphere influenza season2015 25 Feb 2016. Available from: http://www.who.int/influenza/vaccines/virus/recommendations/201509_recommendation.pdf
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