WHO recommendations for influenza virus vaccine composition for the 2019–2020 northern hemisphere season

On 18–20 February 2019 in Beijing, China, the World Health Organization (WHO) agreed on the recommended composition of the quadrivalent influenza vaccine for the northern hemisphere 2019–2020 influenza season: an A/Brisbane/02/2018 (H1N1)pdm09-like virus, a B/Colorado/06/2017-like virus (B/Victoria/2/87 lineage), a B/Phuket/3073/2013-like virus (B/Yamagata/16/88 lineage) and an A(H3N2) virus component to be announced on 21 March 2019. The recommendation for the A(H3N2) component was postponed in light of recent changes in the proportions of genetically and antigenically diverse A(H3N2) viruses to allow more time for the selection of the appropriate virus strain. It is recommended that the influenza B virus component of trivalent vaccines for use in the 2019–2020 northern hemisphere influenza season be a B/Colorado/06/2017-like virus of B/Victoria/2/87-lineage.

The WHO recommendations are based on the viruses currently circulating globally. These viruses are collected through intensive surveillance efforts, where European laboratories and ECDC play a major role.

WHO recommended a change in the (H1N1)pdm09 component compared to the current trivalent and quadrivalent vaccines for the northern hemisphere 2018–2019 influenza season. The majority of circulating A(H1N1)pdm09 viruses have haemagglutinin (HA) gene sequences that belong to the genetically diverse phylogenetic subclade 6B.1A. In Europe and worldwide, predominant viruses also carry amino acid substitution S183P in HA1. Although almost all currently circulating A(H1N1)pdm09 viruses are antigenically similar to the 2018–2019 vaccine virus A/Michigan/45/2015 in haemagglutination inhibition (HI) assays, serological studies showed that a virus component representing this newly emergent subgroup would be more appropriate. The A(H1N1)pdm09 virus component was therefore changed from A/Michigan/45/2015 to A/Brisbane/02/2018.

There is considerable genetic and antigenic diversity of recently circulating A(H3N2) viruses. The majority of circulating A(H3N2) viruses in Europe belong to the phylogenetic subclade 3C.2a1b. However, the number of clade 3C.3a viruses has increased substantially since November 2018 in several geographic regions in Europe and worldwide. The 3C.2a and 3C.3a viruses are antigenically distinct. A low proportion of A(H3N2) viruses was inhibited well by antisera raised against the 2018–2019 egg-propagated vaccine virus A/Singapore/INFIMH-16-0019/2016, while none of the 3C.3a viruses were recognised by the 2018–2019 vaccine virus. The recommendation for the A(H3N2) vaccine component will be announced by WHO in 21 March 2019.

In Europe and globally, both B virus lineages co-circulate in very low numbers. All circulating B/Yamagata/16/88 lineage viruses are well inhibited by antisera raised against B/Phuket/3073/2013. The same component was retained for the quadrivalent 2019–2020 vaccine.

WHO recommended the inclusion of the B/Victoria lineage in the trivalent vaccine for the northern hemisphere 2019–2020 influenza season. All circulating B/Victoria viruses belong to clade 1A, but are genetically and antigenically diverse, with the majority belonging to either the subgroup that carries two amino acid deletions (Δ2) or the one that carries three amino acid deletions (Δ3) in HA. The recommended 2018–2019 vaccine virus B/Colorado/06/2017 that belongs to the Δ2 subgroup was retained for the trivalent and quadrivalent 2019–2020 vaccines.