Flu Vaccine: Flu Shot Facts, Information & Side Effects

flu shot
Seasonal flu shots are recommended for everyone ages 6 months and older, according to the Centers for Disease Control and Prevention.
Credit: AZP Worldwide | Shutterstock

The seasonal flu shot is a yearly vaccine administered to protect against the flu, or influenza.

In the United States, flu shots are recommended for everyone ages 6 months and older, according to the Centers for Disease Control and Prevention.

The flu can be a very serious illness, especially in young children, adults ages 65 and over, those with underlying health conditions, and pregnant women.

The flu shot is the best way to protect yourself and family from the flu, the CDC says.

Strains of the flu virus are constantly changing, so a new flu vaccine is made each year. Scientists make the vaccine before flu season starts by predicting which flu strains are likely to be the most common during the upcoming season.

What kinds of flu shots are there?

Flu shots protect against three or four strains of flu virus. Trivalent flu vaccines protect against two influenza A strains, H1N1 and H3N2, and one influenza B strain. Quadrivalent flu vaccines — offered for the first time in the 2013-2014 flu season — protect against the same strains as the trivalent vaccine, as well as an extra influenza B virus.

In addition to the standard dose flu vaccine given with a needle, flu shots are available in several different forms, including an egg-free version for people ages 18 to 49, a high-dose version for those ages 65 and older, a small-needle version for people ages 18 to 64, and a nasal spray, which is approved for healthy people ages 2 to 49. In the 2013-2014 flu season, all nasal spray doses will protect against four strains of flu virus.


When should you get a flu shot?

Exactly when flu season starts and ends is unpredictable, so health officials recommend that people get their flu shot in early fall, before activity starts to rise. Flu activity typically peaks in January or February.

After vaccination, it takes a person about two weeks to build up immunity against the flu.

People can visit the CDC's HealthMap Vaccine Finder to find flu shot locations, although they should call the location ahead of time to see if they have the vaccine in stock.

How effective is the flu vaccine?

The effectiveness of the seasonal flu vaccine depends upon several factors, including how well the flu strains in the vaccine match the strains in circulation. Some studies show that when strains in the vaccine are a good match with the ones that are circulating, vaccinated individuals are 50 to 60 percent less likely to catch the flu than people who aren't vaccinated.

Flu vaccine effectiveness can also vary depending on the person being vaccinated — the vaccine tends to work best in healthy adults and older children, and less well in older adults.

For instance, a study found that last year's flu vaccine was not very effective in adults ages 65 and over: older people who got the vaccine were just as likely to visit the doctor for flu symptoms as those who did not get the vaccine.

But other studies suggest that individuals who do get sick develop less serve symptoms if they are vaccinated. A study published last year found that people who got the flu shot were less likely to be hospitalized with the flu.

What are the side effects?

According to the CDC, mild side effects from the flu shot include: soreness, redness or swelling at the injection site, low-grade fever, and aches.

Because the viruses in the flu shot are killed, people cannot get the flu from a flu vaccine. Rare but serious side effects can occur, including allergic reactions. Symptoms of serious side effects include: difficulty breathing, swelling around the eyes or lips, hives, racing heart, dizziness and high fever. If you experience serious side effects, you should seek medical care immediately, the CDC says.

For children, side effects from the flu nasal spray can include: runny nose, wheezing, headache, vomiting, muscle aches and fever. For adults, side effects include: runny nose, headache, sore throat and cough.

You should not get the flu vaccine if you have a fever (You should wait until the fever is gone.)

Children younger than 6 months cannot get a flu shot. Those who've had a severe allergic reaction to a flu vaccine in the past should generally not be vaccinated, the CDC says.

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