Influenza vaccine

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.