Influenza vaccines, colloquially known as flu shots, are vaccines that protect against infection by influenza viruses.[14][15] New versions of the vaccines are developed twice a year, as the influenza virus rapidly changes.[14] While their effectiveness varies from year to year, most provide modest to high protection against influenza.[14][16] Vaccination against influenza began in the 1930s, with large-scale availability in the United States beginning in 1945.[17][18]
The vaccines are generally safe, including for people who have severe egg allergies.[25] 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.[14] Influenza vaccines are not recommended in those who have had a severe allergy to previous versions of the vaccine itself.[14][25] 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.[14] 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).[14] The intradermal vaccine was not available during the 2018–2019 and 2019–2020 influenza seasons.[26][27][28]
^World Health Organization (May 2022). "Vaccines against influenza: WHO position paper – May 2022". Weekly Epidemiological Record. 97 (19): 185–208. hdl:10665/354265.
^ abCite error: The named reference pmid34448800 was invoked but never defined (see the help page).
^Organization WH (2019). World Health Organization model list of essential medicines: 21st list 2019. Geneva: World Health Organization. hdl:10665/325771. WHO/MVP/EMP/IAU/2019.06. License: CC BY-NC-SA 3.0 IGO.
^Organization WH (2021). World Health Organization model list of essential medicines: 22nd list (2021). Geneva: World Health Organization. hdl:10665/345533. WHO/MHP/HPS/EML/2021.02.
^"Influenza Virus Vaccine Inactivated". The American Society of Health-System Pharmacists. November 19, 2018. Archived from the original on October 14, 2019. Retrieved October 13, 2019.