Seasonal Flu Safety

What do we know about the safety of influenza vaccines?

Influenza vaccines have been monitored and evaluated for safety for decades. Like any medication, they can cause mild side effects, but serious reactions are rare.

What are the most common side effects of flu shots?

  • Soreness, redness or swelling at the site of injection
  • Fever
  • Aches
  • Nausea

How long do these side effects last?

They typically start soon after the vaccination and last a day or two.

I’ve heard that vaccines can cause fainting.

Fainting can occur after any type of vaccination and is most common among adolescents. By sitting or lying down for 20 minutes after receiving a vaccine, you can minimize the risk of fainting-related injuries.

Is mercury used as a preservative in influenza vaccines?

Influenza vaccine packaged in multi-dose vials contains a small amount of the preservative thimerosal which is used to prevent contamination. Thimerosal contains ethyl mercury which is different from methyl mercury, the form of mercury which causes adverse health effects. In numerous scientific studies, thimerosal has not been shown to cause any health problems, but people who want to avoid thimerosal can still get vaccinated against seasonal influenza. Influenza vaccines packaged in single-dose vials, or pre-filled single dose syringes do not contain thimerosal.

Is thimerosal dangerous to children?

The claim that thimerosal increases the risk of autism has been discredited. In 2004, an expert panel convened by the Institute of Medicine reviewed many studies looking into this theory and found no association between autism and thimerosal. Subsequent studies have reached the same conclusion.

Is it safe for pregnant women to receive vaccines that contain thimerosal?

Yes. In a study of more than 2,000 pregnant women who received influenza vaccines, researchers found no evidence linking thimerosal to pregnancy complications. Pregnancy increases a woman’s risk of complications from influenza, so the benefits of vaccination far outweigh any theoretical risks thimerosal might pose.

Do any influenza vaccines contain adjuvants?

Adjuvants are ingredients added to some vaccines to boost their effectiveness. This year the flu vaccine FluAd contains an adjuvant and is approved for use in people aged 65 years and older.

Do influenza vaccines cause Guillain-Barre´ syndrome (GBS)?

GBS is a medical condition in which the body damages its own nerve cells, causing muscle weakness and sometimes paralysis. There was a very slight increase in GBS among people vaccinated against swine flu in 1976. The risk of GBS is exceedingly rare; studies suggest that influenza vaccines may cause 1 additional case of GBS for every 1 million people vaccinated. The Food and Drug Administration, the Centers for Disease Control and Prevention and others closely monitor reports of serious adverse events, including GBS, among people who get the influenza vaccine each year.

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. The vaccine is given into the muscle which can cause soreness. The needle stick may also cause some soreness.

Why do some people who get vaccinated still get influenza-like illness?

  • The person could have contracted influenza shortly before getting vaccinated or during the two-week period before the vaccine took full effect.
  • The person could have contracted one of the non-influenza respiratory viruses, like the common cold, that circulate during influenza season and cause similar symptoms.
  • The person could have contracted an influenza virus that wasn’t covered by that year’s vaccine. Many different influenza viruses circulate each year, and the seasonal vaccine can’t always prevent all of them.

Can getting a flu shot give me influenza?

No, the virus used in injectable vaccine cannot cause influenza. It has been killed and cannot reproduce anywhere in the body. When researchers compared people receiving flu shots with people getting saltwater shots, the only symptoms unique to the vaccine recipients were soreness in the arm and redness where the shot was given. There were no differences in body aches, fever, cough, runny nose or sore throat.

Who should not get an influenza vaccine?

  • Anyone who has had a serious allergic reaction or other problem after receiving an influenza vaccine in the past
  • People with a moderate or severe illness should wait to be vaccinated until they are better. People with a history of Guillain-Barré (GBS) should speak with their doctor before getting vaccinated.
  • People with a history of serious allergy to eggs should speak with their doctor about getting vaccinated.

What are the signs of a serious allergic reaction to an influenza vaccine?

  • Difficulty breathing
  • Hoarseness or wheezing
  • Swelling around the eyes or lips
  • Hives
  • Paleness or weakness
  • Rapid heartbeat or dizziness

How should I handle a severe reaction to influenza vaccine?

If a person is experiencing significant side effects, call a doctor right away. Write down what happened and the date and time it happened. Ask the doctor, nurse or health department to file a Vaccine Adverse Event Report form. You can download the form at or obtain it by calling 800-822-7967 or 347-396-2400. If the person has signs of a serious allergic reaction, call 911 immediately.

How is the government monitoring the safety of influenza vaccines?

The Centers for Disease Control and Prevention and the Food and Drug Administration closely monitor the safety of all vaccines licensed for use in the United States, including seasonal influenza vaccines. The New York City Health Department is doing its own monitoring to help ensure that any rare side effects are detected as soon as possible.

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