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Flu Shot: Influenza Vaccine and Side Effects

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What types of flu shots or influenza vaccines are available? continued...

Flu vaccines now come in two forms: trivalent vaccines protect against three flu strains (two Influenza A viruses and one Influenza B virus), while quadrivalent vaccines protect against four flu strains (two Influenza A and two Influenza B viruses). The traditional flu shot is available in both trivalent and quadrivalent forms, the high-dose flu shot and the intradermal flu shot only in the trivalent form, and the nasal spray flu vaccine only in the quadrivalent form.

How does the flu shot or influenza vaccine work to prevent flu?

Flu shots and the nasal flu vaccine work by causing antibodies to develop in your body. These antibodies provide protection against infection from the flu virus. This antibody reaction may cause fatigue and muscle aches in some people.

Remember that the flu vaccine cannot cause the flu. Sometimes, people who get vaccinated during flu season catch the flu in the two weeks before the vaccine has a chance to fully work. While it's human nature to see a link between the two events, there's no medical evidence that flu vaccines cause flu or make people susceptible to flu. And even though flu vaccines are not 100% effective -- vaccinated people sometimes get flu infections -- vaccinated people almost always have milder flu than people who weren't vaccinated.

Each year, the flu vaccine contains several different kinds of the virus. The strains chosen are the ones that researchers determine are most likely to show up that year.

Who should get the flu shot?

An annual flu vaccination is recommended for everyone ages 6 months and older. It's particularly advised for high-risk individuals who are more prone to flu complications, such as pneumonia. Those at higher risk for complications include:

  • Children younger than age 5, but especially those younger than 2 years old
  • People age 65 years or older
  • Women who will be pregnant during the flu season
  • People who live in nursing homes
  • Anyone with chronic heart or lung conditions, including asthma, or with any condition that weakens the immune system, such as diabetes or HIV
  • Household caregivers -- including baby sitters -- of any children younger than age 5. This is particularly important for household caregivers of infants younger than age 6 months. (These children are too young to receive the flu vaccine.)
  • Any person in close contact with someone in a high-risk group, such as health care workers and household contacts

To learn more about why all children should receive the flu vaccine, watch CDC's video Children Lost to the Flu. You may also want to visit the Families Fighting Flu web site.

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