Wikipedia
Influenza vaccines, also known as flu shots, are vaccines that protect against influenza. A new version of the vaccine is 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. They decrease the number of missed days of work by a half day on average. Vaccinating children may protect those around them but the effectiveness of the vaccine in those over 65 years old is unknown because the evidence for this group is poor.
Both the World Health Organization and Centers for Disease Control and Prevention recommends nearly all people over the age of 6 months get the vaccine yearly. This is especially true for pregnant women, children between six months and five years of age, those with other health problems, Native Americans, and those who work in healthcare.
The vaccines are generally safe. In children fever occurs in between 5 and 10%, as may muscle pains or feeling tired. In certain years, the vaccine causes Guillain-Barre syndrome in older people in about one per million doses. It should not be given to those with severe allergies to eggs or to previous versions of the vaccine. The vaccines come in both inactive and weakened viral forms. The inactive version should be used for those who are pregnant. They come in forms that are injected into a muscle, sprayed into the nose, or injected into the middle layer of the skin.
Vaccination against influenza began in the 1930s with large scale availability in the United States beginning in 1945. It is on the World Health Organization's List of Essential Medicines, the most important medications recommended for a basic health system. The wholesale price in the developing world is about 5.25 USD per dose as of 2014. In the United States it costs less than 25 USD.