WHO recommendations for influenza virus vaccine composition for the 2016 southern hemisphere season

World Health Organization, Geneva, Switzerland – 24 September 2015.

The World Health Organization (WHO) has agreed on the recommended composition of the trivalent influenza vaccine for the southern hemisphere winter 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.

With these recommendations, two of the vaccine components, the A(H3N2) and B virus, will be changed in the trivalent vaccines in comparison with the equivalent vaccine last year. The reason for the change is that increasing proportions of the circulating A(H3N2) viruses have undergone antigenic drift since the last vaccine recommendation [1]. Furthermore, in Australia and New Zealand, a rapid increase in the proportion of B/Victoria/2/87-lineage viruses was observed from June and they became the predominant lineage by August 2015. All the (sub)types of influenza viruses have been circulating in the southern hemisphere with variable levels of activity. Further antigenic and genetic characteristics of recent seasonal influenza viruses are described below.

Influenza A(H1N1)pdm09 viruses

The A(H1N1)pdm09 viruses remain antigenically homologous and closely related to the earlier vaccine virus A/California/7/2009 and therefore there is no change in the recommendation for this component. The sequence analysis of the haemagglutinin (HA) genes of A(H1N1)pdm09 viruses indicated that most of the recently circulating viruses belonged to genetic clade 6B, which continues to diversify.

Influenza A(H3N2) viruses

The A(H3N2) viruses circulating from February to August 2015 have continued to evolve and fall genetically into two groups (i.e., phylogenetic clades 3C.2 and 3C.3). Among these, there were three genetic sub-clades that circulated. Viruses in sub-clade 3C.2a are now predominant in all regions of the world and the recommendation is to change the A(H3N2) vaccine component to a virus closer to the recently circulating viruses (A/Hong Kong/4801/2014 (H3N2)-like virus).

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/Yamagata lineage in many countries. However, in Australia and New Zealand, a rapid increase in the proportion of B/Victoria/2/87-lineage viruses was observed from June and they became the predominant lineage by August 2015.

The majority of circulating viruses of the B/Yamagata/16/88 lineage fell genetically within two groups, clades 2 and 3, with the majority collected in 2015 falling in clade 3. The recent B/Yamagata viruses were antigenically similar to the previous southern and northern hemisphere vaccine virus of the B/Yamagata/16/88 lineage (B/Phuket/3073/2013-like), and this virus is still recommended for the quadrivalent vaccines and for the countries detecting predominantly B/Yamagata-lineage viruses.

The majority of viruses of the B/Victoria-lineage 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.

The recommendation to change the vaccine composition for the influenza B viruses is based on the current predominance of the B/Victoria-lineage viruses in New Zealand and Australia. The B/Victoria-lineage vaccine component recommendation remains the same for the current northern hemisphere influenza seasonal vaccine [2].

ECDC comment:

The WHO recommendations reflect the currently circulating viruses in the southern hemisphere, and the change of both A(H3N2) and B virus components is based on the available surveillance and virus characterisation data.

The A(H1N1)pdm09 viruses have remained antigenically similar to the A/California/7/2009 virus that is still recommended as the A(H1N1) component.

The continued antigenic drift in the A(H3N2) viruses, and the difficulties of the egg-grown vaccine component A/Switzerland/9715293/2013 (H3N2)-like virus to induce high titers of antibodies, may indicate that the northern hemisphere vaccine for 2015–2016 season may not give an optimal protection against the A(H3N2) virus, especially if the predominance of the subclade 3C.2a viruses, which was observed in the 2014–2015 season, continues in Europe [3]. In the northern hemisphere, both B virus lineages have co-circulated with the predominance of B/Yamagata lineage viruses.  

During the 2014–2015 season, low vaccine effectiveness (VE) was observed, especially against the dominant subtype A(H3N2), [4-6] due to the mismatch between the vaccine components and the circulating viruses. For the B viruses, the B/Phuket/3073/2013-like virus (Yamagata lineage) vaccine component selected for the 2015–2016 northern hemisphere component should protect well against the majority of the B Yamagata lineage viruses. If the Yamagata viruses continue to predominate, like they did during the 2014–2015 season, this vaccine component would be an optimal selection. However, if the Victoria lineage viruses predominate, like they currently do in New Zealand and Australia, then the vaccine component selected for the B viruses for the northern hemisphere would not give an optimal protection.