Influenza vaccine

New versions of the vaccines are developed twice a year, as the influenza virus rapidly changes.^[1] While their effectiveness varies from year to year, most provide modest to high protection against influenza.^[1][2]

The United States Centers for Disease Control and Prevention (CDC) estimates that vaccination against influenza reduces sickness, medical visits, hospitalizations, and deaths.^[3][4] Immunized workers who do catch the flu return to work half a day sooner on average.^[5] Vaccine effectiveness in those over 65 years old remains uncertain due to a lack of high-quality research.^[6][7] Vaccinating children may protect those around them.^[1]

Vaccination against influenza began in the 1930s. It started with large-scale availability in the United States since 1945.^[8][9] It is on the World Health Organization's List of Essential Medicines.^[10]

The vaccines are generally safe; fever occurs in five to ten percent of children vaccinated. 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.^[1] Although most influenza vaccines are produced using egg proteins, they are still recommended as safe for people who have severe egg allergies,^[11] as no increased risk of allergic reaction to the egg-based vaccines has been shown for people with egg allergies.^[12] Vaccines produced using other technologies, notably recombinant vaccines and those based on cell culture rather than egg protein, started to become available from 2012 in the US,^[13] and later in Europe^[14] and Australia.^[12] Influenza vaccines are not recommended in those who have had a severe allergy to previous versions of the vaccine itself.^[1][11] 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.^[1] Depending on the type they can be injected into a muscle, sprayed into the nose, or injected into the middle layer of the skin (intradermal).^[1] The intradermal vaccine was not available during the 2018–2019 and 2019–2020 influenza seasons.^{[15][16][17][18]}