(CDC Report): There are many different flu viruses and they are constantly changing. The composition of U.S. flu vaccines is reviewed annually and updated as needed to match circulating flu viruses. Flu vaccines protect against the three or four viruses (depending on the vaccine) that research suggests will be most common.
For 2020-2021, trivalent (three-component) egg-based vaccines are recommended to contain:
A/Guangdong-Maonan/SWL1536/2019 (H1N1)pdm09-like virus (updated)
A/Hong Kong/2671/2019 (H3N2)-like virus (updated)
B/Washington/02/2019 (B/Victoria lineage)-like virus (updated)
Quadrivalent (four-component) egg-based vaccines, which protect against a second lineage of B viruses, are recommended to contain:
the three recommended viruses above, plus B/Phuket/3073/2013-like (Yamagata lineage) virus.
For 2020-2021, cell- or recombinant-based vaccines are recommended to contain:
A/Hawaii/70/2019 (H1N1)pdm09-like virus (updated)
A/Hong Kong/45/2019 (H3N2)-like virus (updated)
B/Washington/02/2019 (B/Victoria lineage)-like virus (updated)
B/Phuket/3073/2013-like (Yamagata lineage) virus