The 2021-2022 flu shot is a quadrivalent flu vaccine that protects against four different flu viruses. In the U.S., these include two influenza A viruses and two influenza B viruses. According to the FDA, the specific viruses are an A/Victoria/2570/2019 (H1N1) pdm09-like virus, an A/Cambodia/e0826360/2020 (H3N2)-like virus, a B/Washington/02/2019-like virus (B/Victoria lineage), and a B/Phuket/3073/2013-like virus (B/Yamagata lineage).