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Harvard Commentaries
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Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School


Medical Myths, Colds And Flu Medical Myths, Colds And Flu
 

Getting a Flu Shot -- Will it Cause the Flu?


February 27, 2013

By Robert H. Shmerling M.D.

Beth Israel Deaconess Medical Center

Drug Resource Center
8124
Colds And Flu
Getting a Flu Shot -- Will it Cause the Flu?
Getting a Flu Shot -- Will it Cause the Flu?
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Concerns about the safety of vaccinations seem to be growing. One of the most prevalent worries is that the vaccine will cause the disease against which it is supposed to protect.
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InteliHealth
2010-09-16
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Harvard Medical School Commentary
2012-09-08

Last reviewed February 27, 2013
 

Concerns about the safety of vaccinations seem to be growing. One of the most prevalent worries is that the vaccine will cause the disease it's supposed to prevent. For the most common vaccinations, this notion is a myth — the vaccine can't cause the illness it is intended to fight because it contains no biologically active infectious agent.

Consider the seasonal influenza vaccination. Every fall, people at highest risk of influenza and its associated complications — people at least 50 years old, nursing home residents, those with chronic lung diseases, diabetes, asthma, and those with other immune problems — are advised to get immunized against the "flu." Children ages 6 months to 23 months and women who will be in their second or third trimester of pregnancy during flu season also are encouraged to have the routine vaccination. In addition, health care professionals are advised to undergo immunization to reduce the chances of passing influenza to people at risk of severe disease.

However, many people ignore these recommendations because they believe they will contract the infection that the vaccination is meant to prevent.

How Influenza Vaccine Works

Influenza vaccine is produced by growing the virus in eggs. The virus is killed and processed to create the vaccine, which is given by injection under the skin. The body then produces antibodies to the virus over the next two to four weeks. If the immunized person then comes into contact with the influenza virus, the antibodies attack and kill the virus before it has a chance to cause infection. At least, that's the way it's supposed to work.

However, vaccination is not foolproof. During the time it takes for the immune system to produce protective antibodies, you are still vulnerable to infection. In addition, some people don't make enough antibodies in response to the vaccine. And not all strains of influenza are covered by the vaccine — many people think the vaccine will protect against all winter colds, but clearly it can't do that. Despite these few potential disadvantages, the vaccine has no living virus and cannot cause the flu.

While flu virus vaccinations cannot cause the flu, flu vaccines can cause other side effects. In fact, any vaccine can cause trouble. For example, you could have an allergic reaction to the flu vaccine — especially if you have a known allergy to eggs. A rash is most common, but in rare cases, a more severe reaction can compromise breathing or swallowing. For this reason, persons with a known egg allergy are advised against receiving the flu vaccine. Other side effects — reported in a small percentage of people — include redness or pain at the injection site, fever, muscle aches and fatigue. Overall, the incidence of side effects is less than 5% for the seasonal flu vaccination, and the vast majority of side effects are insignificant.

Why Does This Myth Persist?

At least two factors contribute to the persistence of this myth that vaccination can lead to the flu:

    • The timing is coincidental. Flu vaccinations are provided at the start of cold and flu season, and many people can develop a cold or the flu if the vaccine hasn't had time to work. Because of the timing, these people may blame the vaccine for their infection. In fact, common colds have nothing to do with the flu (as they are caused by separate viruses), and carefully performed trials show no greater incidence of colds or influenza infection in people who have just been vaccinated when compared with those who didn't undergo vaccination. Studies also demonstrate that the frequency of influenza in vaccinated people is much lower, confirming the effectiveness of the vaccine.
    • Some vaccines use live virus particles. Unlike influenza vaccines, measles vaccines and some polio vaccines contain tiny amounts of live virus. The live virus is altered so that it rarely causes infection, but it can. Influenza vaccine is killed and processed, so there are no live elements.

The Bottom Line

The seasonal flu shot is not perfect — it's not uniformly effective and does have occasional mild (and rarely severe) side effects. However, experts generally agree that the benefits of vaccination dwarf its risks. If you are a person who has been avoiding vaccination and you are in a risk group for whom vaccination is recommended, you should reconsider. Don't compromise your health.

If you do choose to avoid vaccination, find a better excuse than fear of causing the flu.

Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

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