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Flu Shots (Influenza Vaccine)

What are flu shots?

When you get a flu shot you are injected in the arm with the influenza vaccine. The vaccine helps protect adults and children from getting influenza (flu).

Flu viruses are always present somewhere in the world. In the United States flu outbreaks most often occur each year between October and March.

Influenza causes a cold-like upper respiratory infection. Unlike most colds, however, the flu often causes fever, muscle aches, and dizziness. It can also lead to more serious infections, such as pneumonia. Complications from flu cause up to 36,000 deaths per year in the US. These deaths usually occur among older adults and people who have chronic health problems, such as heart disease and severe asthma.

How do they work?
The formula for the flu vaccine is changed every year according to the recommendations of scientists from the Centers for Disease Control (CDC). They predict which strains of flu virus are most likely to appear the next winter.

The virus in the flu vaccine given as a shot has been inactivated, or "killed." This means you cannot catch the flu from getting a shot. Getting the shot introduces part of the inactive flu virus to your immune system. Your body then reacts by making protective antibodies against the virus.

How effective are they?
Flu shots are usually about 70% effective in preventing flu. Even if you do get the flu, the vaccine helps protect against severe and possibly life-threatening infection. For this reason it is said that the flu shot protects against death from the flu more than from flu itself.

Who should get the flu vaccine?
In general, anyone who wants to reduce their chances of getting the flu can get the flu shot. Some people need to be sure to get the shot every year. They are either people who are at high risk of having serious flu complications or people who live with or care for people at high risk for serious complications. Groups of people who should get the flu vaccine every year are:

  • children 6 months to 18 years old
  • pregnant women
  • people 50 years old or older
  • people with chronic medical problems, such as asthma, heart disease, chronic obstructive pulmonary disease (COPD), and diabetes
  • people who live in nursing homes and other long-term care facilities
  • people whose immune systems may be impaired, such as by HIV, cancer, or an organ transplant, or because they no longer have a working spleen
  • people working in healthcare
  • people who live with or care for others at high risk for complications from flu
  • household contacts and other caregivers of children less than 6 months of age (these children are too young to be vaccinated).
  • people who provide childcare for children under 5 years of age.


Normally, women who are pregnant should get the shot after 13 weeks of pregnancy. However, if they have other chronic medical problems, it is OK to get the shot earlier in the pregnancy.

When there is a shortage of flu vaccine, the recommendations for who should get a shot may change, based on the severity of the shortage.

Who should not get the flu vaccine?
Talk with your healthcare provider before getting a flu shot if you:

  • are sick with a fever
  • have ever had a severe allergic reaction to eggs or to a previous flu shot (eggs are used to help make flu vaccine)
  • had Guillain-Barré syndrome (GBS) within 6 weeks after getting the flu vaccine.

It is OK to get the vaccine if you just have an upper respiratory infection--that is, a cold--without a fever.

Some vaccine manufacturers also recommend not getting a flu shot if you have an allergy to thimerosal. Thimerosal is a preservative used in contact lens solutions and some flu vaccines.

What is FluMist?
A nasal spray called FluMist is another way to get the flu vaccine. Adults can use this nasal spray if they are healthy, not pregnant, and under age 50. It costs more than the shot. As with flu shots, you will need a new dose of FluMist every year. Unlike the shot, FluMist is a live virus vaccine. For this reason pregnant women and people with weakened immune systems, asthma, or some other medical conditions cannot take the nasal spray.

When should I get the flu vaccine?
The flu vaccine becomes effective 1 to 2 weeks after you get the shot. It is best to get the flu shot in October of each year, before the flu season begins. Try to get the shot no later than November, if possible.

Flu seasons can vary from region to region. If you are at high risk from infection and plan to travel to an area where you might be exposed to the flu, make sure you have an up-to-date flu shot before you go on your trip.

What are the side effects?
For people who do not have egg or thimerosal allergies, the flu vaccine has infrequent, minor side effects, such as:

  • soreness, redness, or swelling where the shot was given
  • mild fever
  • aches.

If these problems occur, they start soon after the shot. They may last 1 to 2 days.

Signs of a serious allergic reaction can include breathing problems, hoarseness or wheezing, hives, paleness, weakness, a fast heartbeat, or dizziness. An allergic reaction may occur within a few minutes to a few hours after the shot.

If you think you are having a serious reaction to the flu vaccine, call your healthcare provider right away. Tell them when you received the vaccine and what your symptoms are. Ask if your provider needs to see you.

Developed by RelayHealth.
Published by RelayHealth.
Last modified: 2009-02-12
Last reviewed: 2009-01-23
This content is reviewed periodically and is subject to change as new health information becomes available. The information is intended to inform and educate and is not a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional.