The periodic replacement of virus strains contained in influenza vaccine is necessary because of the constant evolution of influenza viruses, which can change their antigenic and genetic characteristics. The World Health Organization (WHO) recommends the following influenza vaccine composition information.

Northern Hemisphere 2019 – 2020

WHO recommends that egg based quadrivalent vaccines for use in the 2019-2020 northern hemisphere influenza season contain the following:
•    an A/Brisbane/02/2018 (H1N1)pdm09-like virus;
•    an A(H3N2) virus to be announced on 21 March 2019*;
•    a B/Colorado/06/2017-like virus (B/Victoria/2/87 lineage); and
•    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 2019-2020 northern hemisphere influenza season be a B/Colorado/06/2017-like virus of the B/Victoria/2/87-lineage.
* In light of recent changes in the proportions of genetically and antigenically diverse A(H3N2) viruses, the recommendation for the A(H3N2) component has been postponed.
 

More information
WHO (February 2019) Recommended composition of influenza virus vaccines for use in the 2019- 2020 northern hemisphere influenza season
WHO (February 2019) Frequently Asked Questions

WHO (February 2019)    Candidate vaccine viruses and potency testing reagents for development and production of vaccines for use in the northern hemisphere 2019-2020 influenza season

Southern Hemisphere 2019

WHO recommends that egg based quadrivalent vaccines for use in the 2019 southern hemisphere influenza season contain the following:
•    an A/Michigan/45/2015 (H1N1)pdm09-like virus;
•    an A/Switzerland/8060/2017 (H3N2)-like virus;
•    a B/Colorado/06/2017-like virus (B/Victoria/2/87 lineage); and
•    a B/Phuket/3073/2013-like virus (B/Yamagata/16/88 lineage).

WHO recommends that egg based trivalent vaccines for use in the 2019 southern hemisphere influenza season contain the following:
•    an A/Michigan/45/2015 (H1N1)pdm09-like virus;
•    an A/Switzerland/8060/2017 (H3N2)-like virus; and
•    a B/Colorado/06/2017-like virus (B/Victoria/2/87 lineage).

More information
WHO. (September 2018) Recommended composition of influenza virus vaccines for use in the 2019 southern hemisphere influenza - full report 

WHO. (September 2018) Antigenic and genetic characteristics of zoonotic influenza viruses and development of candidate vaccine viruses for pandemic preparedness

(Updated 21 February 2019)