Influenza vaccines, colloquially known as flu shots or flu jab, are vaccines that protect against infection by influenza viruses. New versions of the vaccines are developed twice a year, as the influenza virus rapidly changes. While their effectiveness varies from year to year, most provide modest to high protection against influenza. Vaccination against influenza began in the 1930s, with large-scale availability in the United States beginning in 1945.

Both the World Health Organization and the US Centers for Disease Control and Prevention (CDC) recommend yearly vaccination for nearly all people over the age of six months, especially those at high risk, and the influenza vaccine is on the World Health Organization's List of Essential Medicines. The European Centre for Disease Prevention and Control (ECDC) also recommends yearly vaccination of high-risk groups, particularly pregnant women, the elderly, children between six months and five years, and those with certain health problems.

The vaccines are generally safe, including for people who have severe egg allergies. A common side effect is soreness near the site of injection. Fever occurs in five to ten percent of children vaccinated, and temporary muscle pains or feelings of tiredness may occur. In certain years, the vaccine was linked to an increase in Guillain–Barré syndrome among older people at a rate of about one case per million doses. Influenza vaccines are not recommended in those who have had a severe allergy to previous versions of the vaccine itself. The vaccine comes in inactive and weakened viral forms. The live, weakened vaccine is generally not recommended in pregnant women, children less than two years old, adults older than 50, or people with a weakened immune system. Depending on the type it can be injected into a muscle (intramuscular), sprayed into the nose (intranasal), or injected into the middle layer of the skin (intradermal). The intradermal vaccine was not available during the 2018–2019 and 2019–2020 influenza seasons.

Vaccines are used in both humans and non-humans. The human vaccine is meant unless specifically identified as a veterinary, poultry, or livestock vaccine.

During the worldwide Spanish flu pandemic of 1918, "Pharmacists tried everything they knew, everything they had ever heard of, from the ancient art of bleeding patients, to administering oxygen, to developing new vaccines and serums (chiefly against what we call Hemophilus influenzae – a name derived from the fact that it was originally considered the etiological agent – and several types of pneumococci). Only one therapeutic measure, transfusing blood from recovered patients to new victims, showed any hint of success."

In 1931, viral growth in embryonated hens' eggs was reported by Ernest William Goodpasture and colleagues at Vanderbilt University. The work was extended to the growth of influenza virus by several workers, including Thomas Francis, Jonas Salk, Wilson Smith, and Macfarlane Burnet, leading to the first experimental influenza vaccines. In the 1940s, the US military developed the first approved inactivated vaccines for influenza, which were used during World War II. Hens' eggs continued to be used to produce virus used in influenza vaccines, but manufacturers made improvements in the purity of the virus by developing improved processes to remove egg proteins and to reduce systemic reactivity of the vaccine. In 2012, the US Food and Drug Administration (FDA) approved influenza vaccines made by growing virus in cell cultures, and influenza vaccines made from recombinant proteins were approved in 2013.

The egg-based technology for producing influenza vaccine was created in the 1950s. In the US swine flu scare of 1976, President Gerald Ford was confronted with a potential swine flu pandemic. The vaccination program was rushed, yet plagued by delays and public relations problems. Meanwhile, maximum military containment efforts succeeded unexpectedly in confining the new strain to the single army base where it had originated. On that base, several soldiers fell severely ill, but only one died. The program was canceled after about 24% of the population had received vaccinations. An excess in deaths of 25 over normal annual levels as well as 400 excess hospitalizations, both from Guillain–Barré syndrome, were estimated to have occurred from the vaccination program itself, demonstrating that the vaccine itself is not free of risks. A 2010 study found a significantly enhanced immune response against the 2009 pandemic H1N1 in study participants who had received vaccination against the swine flu in 1976. The 2009 H1N1 swine flu outbreak resulted in the rapid approval of pandemic influenza vaccines. Pandemrix was quickly modified to target the circulating strain and by late 2010, 70 million people had received a dose. Eight years later, the British Medical Journal (BMJ) gained access to early vaccine pharmacovigilance reports compiled by GlaxoSmithKline during the pandemic, which the BMJ reported indicated death was 5.39 times more likely with Pandemrix versus the other pandemic vaccines. However, more thorough and robust later analyses did not establish any increase of fatalities or most other serious adverse effects, with a possible rare exception for narcolepsy.

A quadrivalent flu vaccine administered by nasal mist was approved by the FDA in March 2012. Fluarix Quadrivalent was approved by the FDA in December 2012.