Composition of Influenza Virus vaccines in the 2021 southern 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.

Recommendations
It is recommended that quadrivalent vaccines for use in the 2021 southern hemisphere influenza season contain the following:

Egg-based Vaccines:
- an A/Victoria/2570/2019 (H1N1)pdm09-like virus;
- an A/Hong Kong/2671/2019 (H3N2)-like virus;
- a B/Washington/02/2019 (B/Victoria lineage)-like virus;
- a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus.

Cell- or recombinant-based Vaccines:
- an A/Wisconsin/588/2019 (H1N1)pdm09-like virus;
- an A/Hong Kong/45/2019 (H3N2)-like virus;
- a B/Washington/02/2019 (B/Victoria lineage)-like virus;
- a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus.

It is recommended that trivalent influenza vaccines for use in the 2021 southern hemisphere influenza season contain the following:
Egg-based Vaccines:
- an A/Victoria/2570/2019 (H1N1)pdm09-like virus;
- an A/Hong Kong/2671/2019 (H3N2)-like virus;
a B/Washington/02/2019 (B/Victoria lineage)-like virus.

Cell- or recombinant-based Vaccines:
- an A/Wisconsin/588/2019 (H1N1)pdm09-like virus;
- an A/Hong Kong/45/2019 (H3N2)-like virus;
- a B/Washington/02/2019 (B/Victoria lineage)-like virus.