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Your body's immune system fights off infections.

Vaccines help your body make antibodies before it is infected by a virus. Flu vaccines contain dead or weakened flu viruses. These help your body make antibodies. So when you come into contact with the live flu virus, your body already knows how to fight it.

Flu viruses mutate (change) from year to year. Every year, scientists try to figure out which strains of the flu are most likely to be around the following winter. Then they make a vaccine to fight those types of flu.

There are two types of flu vaccine.

• The first you take by injection. It's made from dead flu viruses that can't hurt you.¹ One or two weeks after you have this shot, your body builds up immunity against the flu. This lasts about a year.²
• The other type of flu vaccine is a nasal spray. It's called FluMist. It contains flu virus that is alive, but very weak. So it helps you build up immunity without getting sick. It can be taken by healthy people between ages 5 and 49.

Usually you take a flu vaccine in the fall. That way, you are safe when the flu comes around in winter.

The flu virus changes from year to year, and the vaccine wears off. So if you need a flu vaccine, you'll need a new one every year.¹

The flu vaccine is made in eggs, so people who are allergic to eggs shouldn't take the vaccine.

Not everyone needs a flu vaccine. For most healthy people, the flu isn't serious, although it can be unpleasant. If you're elderly or have other health problems, the flu can be more dangerous.

In the United States, a flu vaccine is recommended for:^{3 4}

• Anyone over 50
• Healthy children between 6 months and 5 years old
• Healthy children between 6 months and 9 years old who haven't been immunized before
• School-age children who want to reduce their risk of getting flu or passing it to other people
• Adults and children over 6 months with heart or lung problems, a weak immune system, sickle cell disease, diabetes or kidney disease
• Children or teenagers who must take aspirin (the flu can cause a serious illness called Reye's Syndrome in people who take aspirin)
• People who have problems swallowing
• Women who will be pregnant during the flu season
• People who live in rest homes
• Health care workers
• Anyone who comes in to regular contact with, or looks after, children who are younger than 5 years old
• Anyone who has contact with someone who could get serious health problems if they get the flu
• People caring for those with long-term health problems.

If you're not in one of these groups but think you need a flu vaccine, talk to your doctor.

Most people take a flu vaccine as a shot. You can take the nasal spray vaccine if you are healthy and between the ages of 2 and 49.^{4 5} It isn't recommended for:

• Children who are taking aspirin or a similar drug
• People who have problems with their immune system (for example, if you are having chemotherapy or have HIV)
• Anyone with asthma or chronic obstructive pulmonary disease (COPD)
• People with heart or kidney problems, diabetes or sickle cell disease
• Women who will pregnant during the flu season (usually between December and March)
• People who have had Guillain-Barré syndrome, a rare disorder that affects your nervous system.

Children who are under 9 and haven't been vaccinated before will also need a booster vaccine six weeks to 10 weeks later.⁴

Flu vaccines reduce an adult's chance of getting the flu by about 50 percent to 70 percent.⁶ For example, one study looked at adults who were given the vaccine as an injection. About 2 in 100 people who had the vaccine got the flu. This compared with about 9 in 100 people who didn't get the vaccine.

For children, a flu vaccine reduced their chances of getting the flu by about 60 percent.⁷ One study looked at children who were between 2 and 16. About 8 in 100 children who had the vaccine got the flu. This compared with about 26 in 100 children who didn't get the vaccine.

The vaccine doesn't seem to work quite as well in older people. But it does reduce your chance of getting problems that sometimes happen because of the flu, such as pneumonia.⁸ It may also reduce your chances of needing to go to the hospital or dying during the flu season. The benefits are bigger for older people who live in nursing homes.

The flu vaccine only protects against the flu virus. It's still possible to get a bad cold or another illness that's similar to the flu, even after you've had the flu vaccine.⁹ But the flu is usually more serious than these other illnesses, so it makes sense to get protected.

A flu shot won't cause the flu. The viruses in the vaccine are dead and can't do you harm. But the injection may cause a sore arm.⁹ Some people also have muscle aches or a mild fever.²

The nasal spray flu vaccine may cause a sore throat or a runny nose.⁶

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.