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FACT SHEET 

Key Facts about Influenza (Flu) Vaccine 

The single best way to prevent the flu is to get a flu vaccine each fall. 

There are two types of vaccines:

• The “flu shot”—an inactivated vaccine (containing killed virus) that is given with a needle, usually 

in the arm. The flu shot is approved for use in people older than 6 months, including healthy 

people and people with chronic medical conditions. 

• The nasal-spray flu vaccine—a vaccine made with live, weakened flu viruses that do not cause 

the flu (sometimes called LAIV for “Live Attenuated Influenza Vaccine”). LAIV is approved for use 

in healthy people 5 years to 49 years of age who are not pregnant. 

Each vaccine contains three influenza viruses—one A (H3N2) virus, one A (H1N1) virus, and one B virus. 

The viruses in the vaccine change each year based on international surveillance and scientists’ estimations 

about which types and strains of viruses will circulate in a given year. 

About two weeks after vaccination, antibodies that provide protection against influenza virus infection 

develop in the body. 

When to Get Vaccinated 

October or November is the best time to get vaccinated, but you can still get vaccinated in December and 

later. Flu season can begin as early as October and last as late as May. 

Who Should Get Vaccinated 

In general, anyone who wants to reduce their chances of getting the flu can get vaccinated. However, 

certain people should get vaccinated each year. They are either people who are at high risk of having 

serious flu complications or people who live with or care for those at high risk for serious complications. 

People who should get vaccinated each year are: 

1.) People at high risk for complications from the flu:

• People 65 years and older; 

• People who live in nursing homes and other long-term care facilities that house those with long-term 

illnesses; 

• Adults and children 6 months and older with chronic heart or lung conditions, including asthma; 

• Adults and children 6 months and older who needed regular medical care or were in a hospital during 

the previous year because of a metabolic disease (like diabetes), chronic kidney disease, or weakened 

immune system (including immune system problems caused by medicines or by infection with human 

immunodeficiency virus [HIV/AIDS]); 

• Children 6 months to 18 years of age who are on long-term aspirin therapy. (Children given aspirin 

while they have influenza are at risk of Reye syndrome.); 

• Women who will be pregnant during the influenza season; 

• All children 6 to 23 months of age; 
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• People with any condition that can compromise respiratory function or the handling of respiratory 

secretions (that is, a condition that makes it hard to breathe or swallow, such as brain injury or disease, 

spinal cord injuries, seizure disorders, or other nerve or muscle disorders.) 

2.) People 50 to 64 years of age. Because nearly one-third of people 50 to 64 years of age in the United 

States have one or more medical conditions that place them at increased risk for serious flu complications, 

vaccination is recommended for all persons aged 50 – 64 years. 

3.) People who can transmit flu to others at high risk for complications. Any person in close 

contact with someone in a high-risk group (see above) should get vaccinated. This includes all health-care 

workers, household contacts and out-of-home caregivers of children 0 to 23 months of age, and close 

contacts of people 65 years and older.

Who Should Not Be Vaccinated 

There are some people who should not be vaccinated without first consulting a physician. These include: 

• People who have a severe allergy to chicken eggs. 

• People who have had a severe reaction to an influenza vaccination in the past. 

• People who developed Guillain-Barré syndrome (GBS) within 6 weeks of getting an influenza 

vaccine previously. (See www.cdc.gov/flu/about/qa/gbs.htm.) 

• Influenza vaccine is not approved for use in children less than 6 months of age. 

• People who have a moderate or severe illness with a fever should wait to get vaccinated until their 

symptoms lessen. 

Vaccine Effectiveness 

The ability of flu vaccine to protect a person depends on the age and health status of the person getting 

the vaccine, and the similarity or "match" between the virus strains in the vaccine and those in circulation. 

Testing has shown that both vaccines are effective at preventing the flu. 

Vaccine Side Effects (What to Expect) 

Different side effects can be associated with the flu shot and LAIV. 

The flu shot: The viruses in the flu shot are killed (inactivated), so you cannot get the flu from a flu shot. 

Some minor side effects that could occur are: 

• Soreness, redness, or swelling where the shot was given 

• Fever (low grade) 

• Aches 

If these problems occur, they begin soon after the shot and usually last one to two days. Almost all people 

who get influenza vaccine have no serious problems from it. However, a vaccine may rarely cause serious 

problems, such as severe allergic reactions. 

LAIV: The viruses in the nasal-spray vaccine are weakened and do not cause severe symptoms often 

associated with influenza illness. (In clinical studies, transmission of vaccine viruses to close contacts has 

occurred only rarely.) 

In children, side effects from LAIV can include: 

• runny nose 

• headache 

• vomiting 

• muscle aches 

• fever 
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In adults, side effects from LAIV can include: 

• runny nose 

• headache 

• sore throat 

• cough 

For more information, visit www.cdc.gov/flu, or call CDC at 

800-CDC-INFO (English and Spanish) or 888-232-6358 (TTY).