Composition of Influenza Virus vaccines in the 2018-2019 northern hemisphere

Introduction
The periodic replacement of viruses contained in influenza vaccines is necessary in order for the vaccines to be effective due to the constant evolving nature of influenza viruses, including those circulating and infecting humans.

Twice annually, WHO organizes consultations to analyse influenza virus surveillance data generated by the WHO Global Influenza Surveillance and Response System (GISRS), and issues recommendations on the composition of the influenza vaccines for the following influenza season.
Remarks
There was considerable variation in the predominant virus type circulating in different regions during the period September 2017 to January 2018. Influenza B viruses predominated in many countries, while A(H3N2) viruses predominated in some and A(H1N1)pdm09 viruses circulated widely in Africa, Asia, parts of Europe and in the Middle East.

The vast majority of influenza A(H1N1)pdm09 viruses belonged to genetic subclade 6B.1 and were antigenically indistinguishable from the vaccine virus A/Michigan/45/2015.

Influenza A(H3N2) viruses were associated with outbreaks in several countries. The majority of recent viruses were antigenically related to cell culture-propagated A/Hong Kong/4801/2014-like and A/Singapore/INFMH-16-0019/2016-like viruses; they reacted poorly with ferret antisera raised to many egg-propagated clade 3C.2a viruses, but somewhat better to egg-propagated A/Singapore/INFMH-16-0019/2016-like viruses.

Influenza B viruses of the B/Yamagata/16/88 lineage predominated in most regions of the world. Recent B/Yamagata/16/88 lineage viruses were antigenically and genetically closely related to the vaccine virus B/Phuket/3073/2013. Influenza B viruses of the B/Victoria/2/87 lineage were detected in low numbers but a substantial and increasing proportion of these viruses, containing a two amino acid deletion in their HAs, were antigenically distinguishable from the vaccine virus B/Brisbane/60/2008 but closely related to B/Colorado/06/2017.


Recommendations
It is recommended that quadrivalent vaccines for use in the 2018-2019 northern hemisphere influenza season contain the following:

- an A/Michigan/45/2015 (H1N1)pdm09-like virus;
- an A/Singapore/INFIMH-16-0019/2016 (H3N2)-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).

It is recommended that the influenza B virus component of trivalent vaccines for use in the 2018-2019 northern hemisphere influenza season be
- a B/Colorado/06/2017-like virus of the B/Victoria/2/87-lineage.

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