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 2019-2020, trivalent (three-component) vaccines are recommended to contain:
A/Brisbane/02/2018 (H1N1)pdm09-like virus (updated)
A/Kansas/14/2017 (H3N2)-like virus (updated)
B/Colorado/06/2017-like (Victoria lineage) virus
Quadrivalent (four-component) 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.
The World Health Organization (WHO) made the selection of the H1N1 and both B components for 2019-2020 Northern Hemisphere flu vaccines on February 21 and at that time decided to delay the decision on an H3N2 vaccine component. FDA’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) also selected the H1N1 and B components at their first meeting on March 6, but also decided to postpone the selection of the H3N2 component. WHO selected the H3N2 component listed above on March 21, 2019. VRBPAC chose the same H3N2 component for U.S. vaccines on March 22, 2019.