The flu shot is an inactivated vaccine (containing killed virus) that is given with a needle, usually in the arm. Flu vaccine causes antibodies to develop in the body about two weeks after vaccination. These antibodies provide protection against infection from the viruses that are in the vaccine.

The seasonal flu shot protects against four influenza viruses that research indicates will be most common during the upcoming season, including two influenza A viruses and two influenza B viruses. Egg-based, cell-based, and recombinant flu vaccines are also available.

Everyone 6 months of age and older should get a flu shot every season. Vaccination to prevent influenza is particularly important for people who are at high risk of serious complications from influenza:

• Children aged 6 months to 5 years
• Pregnant women
• People 50 years of age and older
• People of any age with certain chronic medical conditions
• People who live in nursing homes and other long-term care facilities

People who live with or care for those at high risk for serious flu complications should get a flu shot every year too:

• Household contacts of persons at high risk for complications from the flu (see above)
• Household contacts and out-of-home caregivers of children less than 6 months of age (these children are too young to be vaccinated)
• Healthcare workers

During flu seasons when vaccine supplies are limited or delayed, the ACIP (Advisory Committee on Immunization Practices) makes recommendations regarding priority groups for vaccination.

Talk with a doctor before getting a flu vaccine if you:

• Have a severe allergy to eggs
• Have had a severe reaction to an influenza vaccination
• Children younger than 6 months of age (influenza vaccine is not approved for this age group)
• People who have a moderate-to-severe illness with a fever (they should wait until they recover to get vaccinated)
• People with a history of Guillain–Barré Syndrome (a severe paralytic illness, also called GBS) that occurred after receiving the influenza vaccine and who are not at risk for severe illness from influenza should generally not receive the vaccine. Tell your doctor if you ever had Guillain-Barré Syndrome. Your doctor will help you decide whether the vaccine is recommended for you.

You can get a flu vaccine at the same time you have a respiratory illness without fever or if you have another mild illness.

Influenza vaccine effectiveness (VE) can vary from year to year and among different age and risk groups. For more information about vaccine effectiveness, visit How Well Does the Seasonal Flu Vaccine Work?

The viruses in the flu shot are killed (inactivated), so you cannot get the flu from a flu shot. The risk of a flu shot causing serious harm, or death, is extremely small. However, a vaccine, like any medicine, may rarely cause serious problems, such as severe allergic reactions. Almost all people who get the influenza vaccine have no serious problems from it.

• Soreness, redness, or swelling where the shot was given
• Fever (low grade)
• Aches

If these problems occur, they begin soon after the shot and usually last one to two days. These side effects are mild and short-lasting, especially when compared to symptoms of influenza infection.

• Life-threatening allergic reactions are very rare. Signs of a serious allergic reaction can include breathing problems, hoarseness or wheezing, hives, paleness, weakness, a fast heartbeat, or dizziness. If they do occur, it is within a few minutes to a few hours after the shot. These reactions are more likely to occur among persons with a severe allergy to eggs because the viruses used in the influenza vaccine are grown in hens' eggs. People who have had a severe reaction to eggs or to a flu shot in the past should not get a flu shot before seeing a physician.
• Guillain-Barré syndrome: Normally, about one person per 100,000 people per year will develop Guillain-Barré syndrome (GBS), an illness characterized by fever, nerve damage, and muscle weakness. In 1976, vaccination with the swine flu vaccine was associated with getting GBS. Several studies have been done to evaluate if other flu vaccines since 1976 were associated with GBS. Only one of the studies showed an association. That study suggested that one person out of 1 million vaccinated persons may be at risk of GBS associated with the vaccine.

More facts about the potential side effects of the influenza vaccine.

• Call a doctor, or get to a doctor right away
• Tell your doctor what happened, the date and time it happened, and when you got the flu shot
• Ask your doctor, nurse, or health department to file a Vaccine Adverse Event Reporting System (VAERS)* form, or call VAERS at 1-800-822-7967