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 2018-2019

WHO recommends 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); 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 2018-2019 northern hemisphere influenza season be a B/Colorado/06/2017-like virus of the B/Victoria/2/87-lineage.

More Information
WHO. (February 2018) Recommended composition of influenza virus vaccines for use in the 2018-2019 northern hemisphere influenza season - full report

WHO. (February 2018) Questions and Answers
Recommended composition of influenza virus vaccines for use in the 2018-2019 northern hemisphere influenza season and development of candidate vaccine viruses for pandemic preparedness

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 1 October 2018)