WHO recommendations for influenza virus vaccine composition for the 2018 southern hemisphere influenza season

ECDC comment

World Health Organization, Geneva, Switzerland report, published 28 September 2017.

The World Health Organization (WHO) has recommended the composition of the trivalent influenza vaccine for the southern hemisphere winter 2018 influenza season as:

  • an A/Michigan/45/2015 (H1N1)pdm09-like virus;
  • an A/Singapore/INFIMH-16-0019/2016 (H3N2)-like virus; and
  • a B/Phuket/3073/2013-like virus (Yamagata lineage).

Quadrivalent vaccines, containing two influenza B viruses, should include the above three viruses and a B/Brisbane/60/2008-like virus (Victoria lineage).

Two components, for influenza A(H3N2) and B viruses of the vaccine recommended for the southern hemisphere, were changed in comparison with the southern hemisphere 2017 influenza season. Further antigenic and genetic characteristics of recent seasonal influenza viruses are described below and in the full report of the recommendation [1].

Influenza A(H1N1)pdm09 viruses

Influenza A(H1N1)pdm09 viruses circulated at low levels in most regions from February to September 2017. The circulating viruses mainly fell into the phylogenetic subclade 6B.1. Almost all recent influenza A(H1N1)pdm09 viruses were antigenically indistinguishable with ferret antisera from the vaccine virus A/Michigan/45/2015, which has been included in the influenza vaccines since the 2017 southern hemisphere influenza season. No change in vaccine component was recommended.

Influenza A(H3N2) viruses

The influenza A(H3N2) viruses predominated in the southern hemisphere and fell genetically into phylogenetic clades 3C.2a and subclade 3C.2a1, with considerable genetic diversification of the haemagglutinin (HA) and neuraminidase (NA) genes. Antigenic characterisation of 3C.2a viruses continued to be technically difficult. Recent A(H3N2) viruses were better inhibited by a ferret antiserum raised against the egg-propagated reference virus, A/Singapore/INFIMH-16-0019/2016 than ferret antisera raised against other egg-propagated A(H3N2) viruses, including previous vaccine component A/Hong Kong/4801/2014. Therefore a change in vaccine component was recommended from A/Hong Kong/4801/2014 to A/Singapore/INFIMH-16-0019/2016.

Influenza B viruses

The influenza B/Yamagata/16/88 and B/Victoria/2/87 lineage viruses have continued to co-circulate across the world. The B/Yamagata lineage viruses have been predominant and therefore a change from Victoria lineage to B/Yamagata lineage was suggested in the trivalent influenza vaccines. Quadrivalent vaccines include both B virus lineages. The majority of the B/Victoria-lineage viruses are closely related antigenically to the earlier vaccine virus B/Brisbane/60/2008 and thus there was no change to the recommendation which remains the B virus recommended for use in quadrivalent vaccines. The majority of B/Yamagata lineage viruses are antigenically closely related to earlier B/Yamagata vaccine virus B/Phuket/3073/2013, and therefore this virus was recommended as the component in the trivalent vaccines.

ECDC Comment

WHO’s recommendations are based on knowledge of the viruses currently circulating worldwide. The recommendations were to change the A(H3N2) and B components of the trivalent influenza vaccines for the southern hemisphere vaccine.

The A/Michigan/45/2015 A(H1N1)pdm09 vaccine virus is close to the circulating A(H1N1)pdm09 viruses in its genetic and antigenic properties and should result in good vaccine effectiveness if A(H1N1)pdm09 viruses circulate widely. As A(H1N1)pdm09 did not circulate widely in the most recent 2016–2017 northern hemisphere influenza season, no vaccine effectiveness results are available for that subtype [2,3].

WHO has recommended changing the A(H3N2) virus component in the influenza vaccines for southern hemisphere 2018 influenza season to A/Singapore/INFIMH-16-0019/2016 which belongs to subclade 3C.2a1. Genetic analyses of circulating A(H3N2) showed that these viruses have undergone considerable genetic diversification of the HA gene from the current vaccine virus A/Hong Kong/4801/2014 (clade 3C.2a) [4]. Results for the 2016–2017 northern hemisphere influenza season, published mid-season, suggested 23–44% vaccine effectiveness against the recently circulating A(H3N2) viruses which were predominantly of clade 3C.2a [2,3]. The vaccine component for the 2016–2017 northern hemisphere season for A(H3N2) was A/Hong Kong/4801/2014 (clade 3C.2a). The vaccine strain for the upcoming winter season in the northern hemisphere remains the same and this does not correspond ideally with the A(H3N2) viruses currently circulating across the world [1]. Therefore, it is expected that the vaccine effectiveness against this subtype in the northern hemisphere 2017–2018 influenza season will be suboptimal, as anticipated by WHO [5], if the circulating strains resemble those circulating in the southern hemisphere.

Globally, both B virus lineages continue to co-circulate, with B/Yamagata lineage being predominant [1]. WHO recommends the B/Yamagata lineage in the trivalent vaccine for the southern hemisphere 2018 influenza season. However, in the northern hemisphere, B/Brisbane/60/2008-like virus (Victoria lineage) is still included in the trivalent vaccine for the 2017–2018 influenza season. As the trivalent vaccine is the most widely used vaccine in the northern hemisphere, a suboptimal vaccine effectiveness against the B viruses is expected, if the B/Yamagata viruses continue to be predominant. On the other hand, trivalent vaccines containing B/Victoria/2/87 lineage viruses may provide some cross-protection against B/Yamagata/16/88 lineage viruses [5]. A limited proportion of B/Victoria circulating viruses are antigenically different from the vaccine component. In the event that these viruses spread to the northern hemisphere during the season, a suboptimal vaccine effectiveness can be expected.

As all influenza (sub)types continued to co-circulate globally from February to September 2017, it is impossible to predict the proportions of circulating viruses next season and the expected protection using the recommended vaccine components. The technical difficulties in virus culture, characterisation and egg-propagation of influenza A(H3N2) viruses continue to make the selection of new vaccine components difficult.



  1. World Health Organization. Recommended composition of influenza virus vaccines for use in the 2018 southern hemisphere influenza season2017 02 Oct 2017. 
  2. Kissling E, Rondy M, I-MOVE/I-MOVE+ study team. Early 2016/17 vaccine effectiveness estimates against influenza A(H3N2): I-MOVE multicentre case control studies at primary care and hospital levels in Europe. Euro Surveill. 2017;22(7):30464.
  3. Skowronski DM, Chambers C, Sabaiduc S, Dickinson JA, Winter AL, De Serres G, et al. Interim estimates of 2016/17 vaccine effectiveness against influenza A(H3N2), Canada, January 2017. Euro Surveill. 2017;22(6).
  4. Melidou A, Broberg E. Predominance of influenza A(H3N2) virus genetic subclade 3C.2a1 during an early 2016/17 influenza season in Europe - Contribution of surveillance data from World Health Organization (WHO) European Region to the WHO vaccine composition consultation for northern hemisphere 2017/18. Vaccine. 2017;35(37):4828-35.
  5. World Health Organization. Questions and Answers. Recommended composition of influenza virus vaccines for use in the southern hemisphere 2018 influenza season and development of candidate vaccine viruses for pandemic preparedness 2017 [2 October 2017].