Flu Shot: The Vaccine and Its Side Effects

A simple vaccine is the best way to protect yourself from the flu. It may sound like a minor illness, but it can cause big problems for some people. It can even be deadly.

Learn the facts about the vaccine so you and your family can stay flu-free.

When Should I Get It?

Peak flu season can start as early as October and run through May. The best time to get a flu shot is as soon as it's available, usually in September or October. It takes about 2 weeks for the vaccine to take effect. You can still get a shot in December or later, but the earlier you get it, the higher your odds of staying well.

What Types of Vaccine Can I Get?

There are several:

  • The traditional flu shot is an injection into your arm muscle. It’s made from pieces of inactive flu viruses.
  • An egg-free flu shot, unlike other types, isn’t grown inside eggs. It’s an option for people over age 18 who have severe egg allergies.
  • The high-dose flu shot is for people ages 65 and older. They may need a stronger dose to get the same protection.
  • An intradermal flu shot uses a tiny needle that only goes skin deep. It’s for people ages 18 to 64.
  • The nasal spray flu vaccine (FluMist) doesn’t use a needle. It’s made from live but weakened flu viruses. Another name for it is LAIV (live attenuated influenza vaccine). It’s for healthy people ages 2 to 49 who aren’t pregnant, who are not allergic to the flu vaccine or any of its ingredients, and who don’t have weak immune systems. It should not be used for the 2016-2017 flu season. Check with your doctor to make sure it’s right for you.

Some vaccines protect against more than one flu virus strain. Trivalent vaccines work against three strains; quadrivalent vaccines fight four types of flu. The traditional flu shot comes in both forms. High-dose only comes in the trivalent form.


How Does It Work?

The shots prompt your body to get ready to fight an infection from the flu virus. It helps you create tools, called antibodies, to fight the virus when you're exposed to it.

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 the flu shot cannot 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 likely be milder than if you skip the vaccine.

Who Should Get It?

The CDC recommends that everyone 6 months and older get immunized against the flu unless there's a medical reason not to.

The CDC says the vaccine is especially important for people who are at high risk of serious complications if they get the flu, including:

  • Kids younger than age 5, but in particular those under age 2
  • People 65 or older
  • Pregnant women
  • People who live in nursing homes or other kids of long-term care facilities
  • Adults and kids with diseases of the lungs (like asthma), heart, kidneys, liver, blood, or metabolism (like diabetes)
  • Anyone under age 19 who gets long-term aspirin therapy
  • American Indians and Alaskan natives

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 your flu vaccine. It is 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 usually OK to get vaccinated, but talk to your doctor first.

Are There Side Effects?


Yes. You might have a mild fever and feel tired or achy after you get the shot. Some people also have soreness, redness, or swelling where the needle went in. These problems aren't serious and won't last long.

Serious side effects are rare. If you do get them, it should be within a few minutes to a few hours after you get the shot. Call 911 and get emergency help right away if you have trouble breathing, hives, feel weak or dizzy, or have a fast heartbeat after you get it.

Side effects in children can include:

  • Runny nose 
  • Headache 
  • Vomiting 
  • Muscle aches
  • Fever

Side effects in adults can include:

  • Runny nose
  • Headache 
  • Sore throat 

Who Can Get the Nasal Spray?

FluMist is only for healthy people ages 2 to 49,but if it’s not available, kids should get the traditional flu shot rather than wait.  It should not be used in the 2016-2017 flu season.

The nasal spray vaccine contains weakened strains of live viruses. These people shouldn’t take it:

  • Pregnant women
  • Children under age 5 who have wheezing that comes and goes
  • Children and teens who take aspirin 


WebMD Medical Reference Reviewed by Jennifer Robinson, MD on August 10, 2016



CDC: "Estimating Seasonal Influenza-Associated Deaths in the United States: CDC Study Confirms Variability of Flu."


American Lung Association.

National Institute of Allergy and Infectious Diseases: "Flu (Influenza): Prevention."

National Jewish Medical and Research Center: "Is it better to use a flu inhaler or get a flu shot?"

American Lung Association: "Cold and Flu Guidelines: Influenza."

FDA: "Influenza: Vaccination Still the Best Protection."

MedlinePlus: "Flu," "Common Cold."


CDC: “Key Facts About Seasonal Flu Vaccine.”

Flu Near You web site.

News release, American College of Allergy, Asthma and Immunology.

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