Flu, also called influenza, is an infection of the nose, throat and lungs, which are part of the respiratory system. The flu is caused by a virus. Most people with the flu get better on their own. But influenza can cause serious complications for some groups of people.

Getting a flu vaccine is the best way to prevent the flu and its complications for almost everyone. The flu vaccine can lower the risk of getting the flu. It also can lower the risk of having serious illness from the flu and needing to stay in the hospital or dying of the flu.

This year's seasonal flu vaccines each provide protection against three influenza viruses that researchers expect to be most common this flu season.

For almost everyone, a flu vaccine can lower the risk of the flu and its complications. You may be able to get your vaccine at your healthcare professional's office. Other places to check are pharmacies, health centers, and vaccine clinics at your school or workplace.

The flu vaccine is often available in the U.S. around September. The viruses that cause flu spread at high levels during certain times of the year in the Northern and Southern hemispheres. These are called flu seasons. In North America, flu season usually runs between October and May.

It takes about six months for pharmaceutical makers to produce the flu vaccine. In the United States, shipments often begin in late summer.

Once you get a flu vaccine, it takes up to two weeks to build immunity. So it's usually best for people in the United States to get the flu vaccine in September or October. But you can benefit from the vaccine even if you don't get it until later in flu season.

Flu viruses change quickly. Last year's vaccine may not protect you from this year's viruses. New flu vaccines are released every year to keep up with rapidly changing flu viruses.

When you get vaccinated, your immune system makes antibodies to protect you from the viruses included in the vaccine. But antibody levels may decline over time, which is another reason to get a flu shot every year.

The flu vaccine is available as a shot given with a needle, as a jet injection or as a nasal spray.

For older children and adults, the flu shot is usually given in a muscle in the arm. Younger children may get the flu shot in a thigh muscle.

In the U.S., if you are an adult under the age of 65, you can choose to get your vaccine with a device called a jet injector. Instead of a needle, this device uses a high-pressure, narrow stream of fluid to give the vaccine through the skin.

The nasal spray vaccine is approved for people between 2 and 49 years old. The nasal flu vaccine isn't recommended for some people, including:

• People who have severe asthma or are currently wheezing.
• People who had a severe allergic reaction to a flu vaccine in the past.
• Pregnant people.
• Young people who take aspirin or a salicylate-containing medicine.
• People with weakened immune systems and people who are caregivers or close contacts of people with weakened immune systems.
• Children between ages 2 and 4 years old diagnosed with asthma or wheezing in the past 12 months.
• People who recently took antiviral medicine for the flu.
• People with a cerebrospinal fluid leak or the potential for a leak, as with a cochlear implant.

Check with your healthcare team to see if you need to be cautious about getting a nasal spray flu vaccine.

Annual flu vaccines are available in the U.S. for people age 6 months and older. These vaccines are often recommended for everyone who doesn't have a medical reason to avoid the vaccine.

Getting a flu vaccine lowers:

• The risk of getting the flu. If the vaccine is given later in pregnancy, the flu vaccine helps protect a newborn from the flu too.
• The risk of having serious illness from the flu and needing to stay in the hospital due to the flu.
• The risk of dying of the flu.

High risk groups include:

• Adults older than age 65.
• People in nursing homes or long-term care.
• Young children, especially those age 12 months or younger.
• People who plan to be pregnant, are pregnant or recently gave birth during flu season.

In the United States, some groups have a higher risk of needing treatment in the hospital for flu. Those groups include people who are Native American or Alaska Native, Black, or Latino.

Other groups at high risk of flu complications include people with:

• Weakened immune systems.
• A body mass index (BMI) of 40 or higher.
• Conditions that affect the nervous system or how the brain processes information.

And people with certain medical conditions have a higher risk of flu complications, such as:

• People who have chronic illnesses, such as asthma, heart disease, kidney disease, liver disease and diabetes.
• People who have had a stroke.
• People who are younger than 20 years of age and are receiving long-term aspirin therapy.

Vaccines called high-dose or adjuvanted flu vaccines can help people have a stronger immune response against influenza. People over age 65 can get these vaccines. These vaccines also are recommended for people age 18 and older who have a solid organ transplant and take medicine to weaken their immune response.

Children between 6 months and 8 years may need two doses of the flu vaccine, given at least four weeks apart, the first time they get a flu vaccine. After that, they can get single annual doses of the flu vaccine. Check with your child's healthcare professional.

Also, check with your healthcare team before getting a flu vaccine if you had a serious reaction to a previous flu vaccine. People who have had Guillain-Barre syndrome also should check with a healthcare professional before getting the flu vaccine. And if you feel sick when you go to get the shot, check with your healthcare professional to see if you should delay getting the vaccine.

If you have an egg allergy, you can receive any of the flu vaccines, anywhere the vaccine is offered.

No, the flu vaccine can't give you the flu. And the flu vaccine does not make you more likely to get another respiratory infection based on available evidence.

But you might develop flu-like symptoms — despite getting a flu vaccine — for many reasons, including:

• Reaction to the vaccine. Some people have muscle aches and a fever for a day or two after receiving a flu vaccine. This may be a side effect of your body's production of protective antibodies.
• The two-week window. It takes about two weeks for the flu shot to take full effect. If you're exposed to the influenza virus shortly before or during that time period, you might catch the flu.
• Mismatched flu viruses. In some years, the influenza viruses used for the vaccine don't match the viruses spreading during the flu season. If this occurs, your flu shot will be less effective. But it may still offer some protection.
• Other illnesses. Many other illnesses, such as COVID-19, also have flu-like symptoms. So you may think you have the flu when you actually don't.

How well the flu vaccine works to protect you from the flu can vary. Reviews of flu season data since 2009 find that vaccine protection ranges from a low of 22% to a high of 56%. Looking at the U.S. population, that means millions of fewer illnesses and visits to a healthcare professional. Flu vaccine also lowers the number of people with an illness who need hospital care or who die of influenza.

The flu vaccine is your best defense against the flu. But you can take other steps to help protect yourself from the flu and other viruses, including COVID-19.

Follow these standard precautions:

• Wash your hands. Wash your hands well and often with soap and water for at least 20 seconds. If soap and water aren't available, use an alcohol-based hand sanitizer with at least 60% alcohol. Make sure friends and family that you're around regularly, especially kids, know the importance of hand-washing.
• Avoid touching your face. Keeping your hands away from your eyes, nose and mouth helps keep germs away from those places.
• Cover your coughs and sneezes. Cough or sneeze into a tissue or your elbow. Then wash your hands.
• Clean surfaces. Regularly clean often-touched surfaces to prevent spread of infection from touching a surface with the virus on it and then your face.
• Avoid crowds. The flu spreads easily wherever people gather — in child care centers, schools, office buildings and auditoriums and on public transportation. By avoiding crowds during peak flu season, you lower your chances of infection.

Also avoid anyone who is sick.

If you're sick, stay home until you feel better and have had no fever for a full 24 hours, and you haven’t taken medicine for fever during that time. If your fever returns or you start to feel worse, stay apart from others until your symptoms improve and you are fever-free without medicine for 24 hours. Doing so will lower your chance of infecting others.

The flu vaccine will be available as a shot, also called an injection, or as a nasal spray.

The nasal spray vaccine is approved for people between 2 and 49 years old. The nasal flu vaccine isn't recommended for some people, including:

• People who had a severe allergic reaction to a flu vaccine in the past.
• Pregnant people.
• Children age 17 years or younger who are taking aspirin or a salicylate-containing medicine.
• People with weakened immune systems and caregivers or close contacts of people with weakened immune systems.
• Children between ages 2 and 4 years old diagnosed with asthma or wheezing in the past 12 months.
• People who recently took antiviral medicine for the flu.
• People with a cerebrospinal fluid leak or the potential for a leak, as with a cochlear implant.

Check with your healthcare team to see if you need to be cautious about getting a nasal spray flu vaccine.

You also can get the flu vaccine as a shot that's usually given in a muscle in the arm. If you're an adult under 65, you can choose to get your vaccine with a jet injector device, which uses a high-pressure, narrow stream of fluid to go through the skin instead of a needle.