Flu Shot: Influenza Vaccine and Side Effects

Brian King10/21/14

Thinking about getting a flu shot or influenza vaccine this year? The influenza vaccine is the best way to prevent the flu, and flu prevention should be a goal for everyone.

According to the CDC, up to 20% of Americans get the flu each year. More than 200,000 people in the U.S. are hospitalized each year, and somewhere between 3,000 and 49,000 deaths are flu-related. These statistics would decrease if more people took advantage of the opportunity to prevent flu with an influenza vaccine or flu shot.

 When should I get a flu shot?

Because the peak flu season may begin as early as October and run through May, the best time to get a flu shot is in September or October. It takes about two weeks for the flu shot to be most effective. You can still get an influenza vaccine in December or later -- flu season lasts well into spring -- but the earlier in flu season you get it, the better your odds of staying flu free.

What types of flu shots or influenza vaccines are available?

There are several types of flu vaccine:

  • The traditional flu shot. Injected into the muscle, it contains flu-virus particles that stimulate anti-flu immunity but can't cause the flu.
  • Egg-free flu shot. Unlike traditional flu vaccines, this one is not grown inside eggs. Doctors say all but those with the most severe allergic reactions to eggs can get a traditional shot with few side effects.   
  • The high-dose flu shot. Approved for people age 65 and older, the ingredients are the same as the regular flu shot, but the dose is higher, as the aging immune system needs more stimulus to produce adequate immunity.
  • An intradermal flu shot. Approved for people ages 18 to 64, the shot uses a tiny needle that only goes skin deep. It contains the same flu-virus particles as the traditional flu shot.
  • Nasal spray flu vaccine. Approved for some people ages 2 to 49, this vaccine, called LAIV for live attenuated influenza vaccine, contains a live, weakened flu virus. Clinical trials show that it cannot cause the flu. For in-depth information on this vaccine, see WebMD's What is FluMist?

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 fatigueand 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.

Who should talk to their doctors before getting a flu shot?

According to the CDC, you should talk to your doctor before getting a flu shot or influenza vaccine if:

  • You have had an allergic reaction to a flu shot in the past
  • You have previously had Guillain-Barre syndrome that occurred after receiving the influenza vaccine; Guillain-Barre syndrome is a disorder in which the body's immune system attacks part of the nervous system.
  • You are ill; if you have a fever, talk to your health care provider about getting the shot later. If you have a mild illness with no fever, it's OK to get a flu shot.

It's long been advised that people with allergies to eggs should not get the flu shot. However, the American College of Allergy, Asthma and Immunology says the vaccine contains such a low amount of egg protein that it's unlikely to cause an allergic reaction in those with an egg allergy. If you have a severe egg allergy (anaphylaxis), talk to your doctor before getting the flu vaccine. Also, as mentioned above, flu vaccines that do not contain eggs are available. 

What are some flu shot side effects?


You can experience low-grade fever and aching after getting any flu vaccine.  After getting a flu shot some people can also experience soreness, redness, or swelling where the shot was given.

Serious side effects from the flu vaccine are very rare. If they do occur, it's within a few minutes to a few hours after getting the shot. 

It is important to note that the benefits of getting a flu shot far outweigh the risk of flu shot side effects.


Who can receive the nasal influenza vaccine?

The FluMist influenza vaccine is only recommended for healthy individuals ages 2 to 49. The CDC now recommends the nasal spray vaccine for healthy children 2 through 8 years old when it is available.

Because FluMist influenza vaccine contains weakened strains of live virus, it is not recommend for the following groups:

  • Pregnant women
  • Children under age 5 who have recurrent wheezing
  • Children and teens receiving aspirin
  • Anyone with a medical condition that places him or her at high risk for complications from influenza, including people with chronic heart or lung disease (such as asthma), diabetes, kidney disease, or a weakened immune system



What are the side effects of FluMist influenza vaccine?

Side effects from FluMist nasal spray in children can include:

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

Side effects from FluMist nasal spray in adults can include:

  • Runny nose
  • Headache
  • Sore throat