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Cold, Flu, & Cough Health Center

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Flu Shot: The Vaccine and Its Side Effects

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How Does It Work? continued...

Doctors tweak the vaccine each season. They choose strains based on the ones they think are most likely to show up that year.

The vaccine itself doesn’t cause the flu. But it does take about 2 weeks to start working. Some people get it, then catch the virus before their body is ready to fight it. It's human nature to see a link between the two events, but there's no proof that the vaccines cause the flu or make you more likely to get it. They don’t work all the time. You can get sick even if you get one, but your illness will be milder than if you skip the vaccine.

Who Should Get It?

Everyone 6 months old and older should get the vaccine every year. It's really important for people who are more likely to have problems that come along with the flu, like pneumonia. People most at risk include:

  • Children younger than age 5, especially those younger than 2
  • People ages 65 and older
  • Women who will be pregnant during flu season
  • People who live in nursing homes
  • Anyone with long-lasting heart or lung conditions, including asthma, or with any health problem that weakens the immune system, like diabetes or HIV
  • Caregivers, including babysitters, of any children younger than 5. It’s especially important for people who care for infants younger than 6 months. (These children are too young to get the flu vaccine.)

Any person in close contact with someone in a high-risk group, such as people in their households or health care workers

Should I Talk to My Doctor First?

Some people should make sure it’s OK to get vaccinated. Ask your doctor first if:

  • You’ve had an allergic reaction to a flu shot in the past.
  • You’ve had Guillain-Barre syndrome that happened after you got the flu vaccine. That’s a disorder in which your body's immune system attacks part of your nervous system.
  • You’re very ill. If you have a mild illness, it's OK to get vaccinated. Otherwise, talk to your doctor.
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