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Seasonal Flu Shot Q&A

What is the flu shot?

The flu shot is an inactivated vaccine (containing killed virus) that is given with a needle, usually in the arm. It contains three influenza viruses. The three vaccine strains – one A (H3N2) virus, one A (H1N1) virus, and one B virus – are representative of the influenza vaccine strains recommended for that year. Viruses for the flu shot are grown in eggs.

Who should get a flu shot?

In general, anyone who wants to reduce their chances of getting the flu can get vaccinated. However, certain people should get vaccinated each year. They are either people who are at high risk of having serious flu complications or people who live with or care for those at high risk for serious complications. During flu seasons when vaccine supplies are limited or delayed, the ACIP makes recommendations regarding priority groups for vaccination.


People who should get vaccinated each year are:

1. Children aged 6 months up to their 19th birthday
2. Pregnant women
3. People 50 years of age and older
4. People of any age with certain chronic medical conditions
5. People who live in nursing homes and other long-term care facilities
6. People who live with or care for those at high risk for complications from flu, including

a. Health care workers
b. Household contacts of persons at high risk for complications from the flu

Use of the Nasal Spray Flu Vaccine

It should be noted that vaccination with the nasal-spray flu vaccine is always an option for healthy* persons aged 2-49 years who are not pregnant.

Who should not get a flu shot?

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

1. Have ever had a severe allergic reaction to eggs or to a previous flu shot or
2. Have a history of Guillain-Barré syndrome (GBS).

If you are sick with a fever when you go to get your flu shot, you should talk to your doctor or nurse about getting your shot at a later date. However, you can get a flu shot at the same time you have a respiratory illness without fever or if you have another mild illness.

How effective is the flu shot?

With the flu shot, when the "match" between vaccine and circulating strains is close, the vaccine prevents influenza in about 70%-90% of healthy persons younger than age 65 years. Among elderly persons living outside chronic-care facilities (such as nursing homes) and those persons with long-term (chronic) medical conditions (such as asthma, diabetes, or heart disease), the flu shot is 30%-70% effective in preventing hospitalization for pneumonia and influenza. Among elderly nursing home residents, the flu shot is most effective in preventing severe illness, secondary complications, and deaths related to the flu. In this population, the shot can be 50%-60% effective in preventing hospitalization or pneumonia and 80% effective in preventing death from the flu.

What are the risks from getting a flu shot?

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 influenza vaccine have no serious problems from it.


What are the side effects that could occur?

• 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.


Can severe problems occur?

• Life-threatening allergic reactions are very rare. Signs of 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 potential side effects of the influenza vaccine can be found in Prevention & Control of Influenza - Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2007 Jul 13;56(RR06):1-54. Also available as PDF (720K).

What should I do if I have had a serious reaction to 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.

Can the flu shot give me the flu?

No, the flu shot cannot cause flu illness. The three influenza viruses contained in the flu vaccine are each inactivated (killed), which means they cannot cause infection. Flu vaccine manufacturers kill the viruses used in the vaccine during the process of making vaccine, and batches of flu vaccine are tested to make sure they are safe. In randomized, blinded studies, where some people got flu shots and others got saltwater shots, the only differences in symptoms was increased soreness in the arm and redness at the injection site among people who got the flu shot. There were no differences in terms of body aches, fever, cough, runny nose or sore throat.

More information about these studies is available at:

• Carolyn Bridges et al. (2000). Effectiveness and cost-benefit of influenza vaccination of healthy working adults: A randomized controlled trial. JAMA. 284(13):1655–1663.

• Kristin Nichol et al. (1995). The effectiveness of vaccination against influenza in healthy working adults. New England Journal of Medicine. 333(14): 889-893.

Why do some people not feel well after getting the flu shot?

The most common side effect of the flu vaccine in adults is soreness at the spot where the shot was given, which usually lasts less than two days. The soreness is often caused by a person’s immune system making protective antibodies to the killed viruses in the vaccine. These antibodies are what allow the body to fight against flu. The needle stick may also cause some soreness at the injection site. According to the Advisory Committee on Immunization Practices (ACIP), rare symptoms include fever, muscle pain, and feelings of discomfort or weakness. If these problems occur, they are very uncommon and usually begin soon after the shot and last 1-2 days.

What about people who get a flu vaccine and still get sick with flu-like symptoms?

There are several reasons why someone might get flu-like symptoms even after they have been vaccinated against the flu.

1. People may be exposed to an influenza virus shortly before getting vaccinated or during the two-week period that it takes the body to gain protection after getting vaccinated. This exposure may result in a person becoming ill with flu before the vaccine begins to protect them.

2. People may become ill from other (non-flu) viruses that circulate during the flu season, which can also cause flu-like symptoms (such as rhinovirus).

3. A person may be exposed to an influenza virus that is not included in the vaccine. There are many different influenza viruses. For more information, see Influenza (Flu) Viruses.

4. Unfortunately, some people can remain unprotected from flu despite getting the vaccine. This is more likely to occur among people that have weakened immune systems. However, even among people with weakened immune systems, the flu vaccine can still help prevent influenza complications. For more information about the effectiveness of the flu vaccine, see How Well Does the Seasonal Flu Vaccine Work?

Influenza vaccine provides the best protection available from flu – even when the vaccine does not closely match circulating flu strains, and even when the person getting the vaccine has a weakened immune system. Vaccination can lessen illness severity and is particularly important for people at high risk for serious flu-related complications and for close contacts of high-risk people.

* "Healthy" indicates persons who do not have an underlying medical condition that prediposes them to influenza.

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