In the 2019–2020 influenza season all regular-dose flu shots and all recombinant influenza vaccine in the United States are quadrivalent.In November 2019, the FDA approved Fluzone High-Dose Quadrivalent for use in the United States starting with the 2020–2021 influenza season.In February 2020, the FDA approved Fluad Quadrivalent for use in the United States. In July 2020, the FDA approved both Fluad and Fluad Quadrivalent for use in the United States for the 2020–2021 influenza season. Medical uses
The U.S Centers for Disease Control and Prevention (CDC) recommends the flu vaccine as the best way to protect people against the flu and prevent its spread. The flu vaccine can also reduce the severity of the flu if a person contracts a strain that the vaccine did not contain. It takes about two weeks following vaccination for protective antibodies to form.A 2012 meta-analysis found that flu vaccination was effective 67 percent of the time; the populations that benefited the most were HIV-positive adults aged 18 to 55 (76 percent), healthy adults aged 18 to 46 (approximately 70 percent), and healthy children aged six months to 24 months (66 percent). The influenza vaccine also appears to protect against myocardial infarction with a benefit of 15–45%. Effectiveness
A vaccine is assessed by its efficacy – the extent to which it reduces risk of disease under controlled conditions – and its effectiveness – the observed reduction in risk after the vaccine is put into use.
Recent WHO seasonal influenza vaccine composition recommendations:

2020–2021 northern hemisphere
The composition of virus vaccines for use in the 2020–2021 Northern Hemisphere influenza season recommended by the World Health Organization on February 28, 2020, is:for egg-based:

an A/Guangdong-Maonan/SWL1536/2019 (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 virusfor cell- or recombinant-based:
an A/Hawaii/70/2019 (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 virusThe WHO recommends that trivalent vaccines use as their influenza B virus a B/Washington/02/2019 (B/Victoria lineage)-like virus. United StatesThe Vaccines and Related Biological Products Advisory Committee (VRBPAC) of the Food and Drug Administration (FDA) recommended that the quadrivalent formulation of egg-based influenza vaccines for the US 2020–2021 influenza season contain the following:
an A/Guangdong-Maonan/SWL1536/2019 (H1N1)pdm09-like virus;
an A/HongKong/2671/2019 (H3N2)-like virus;
a B/Washington/02/2019-like virus (B/Victoria lineage);
a B/Phuket/3073/2013-like virus (B/Yamagata lineage).The committee recommended that the quadrivalent formulation of cell- or recombinant-based influenza vaccines for the US 2020–2021 influenza season contain the following:
an A/Hawaii/70/2019 (H1N1)pdm09-like virus;
an A/HongKong/45/2019 (H3N2)-like virus;
a B/Washington/02/2019-like virus (B/Victoria lineage);
a B/Phuket/3073/2013-like virus (B/Yamagata lineage)For trivalent influenza vaccines for use in the US for the 2020–2021 influenza season, depending on the manufacturing method of the vaccine, the committee recommended that the A(H1N1)pdm09, A(H3N2) and B/Victoria lineage viruses recommended above for the quadrivalent vaccines be used.