(MayoClinic.com) This year's annual flu shot will offer protection against H1N1 flu (swine flu) virus, in addition to two other influenza viruses that are expected to be in circulation this fall and winter.

Influenza is a respiratory infection that can cause serious complications, particularly to young children and to older adults. Flu shots are the most effective way to prevent influenza and its complications. The Centers for Disease Control and Prevention (CDC) now recommends that everyone 6 months of age or older be vaccinated annually against influenza.

Here are the answers to common questions about flu shots.

Because the flu vaccine is produced by private manufacturers, its availability depends on when production is completed. For the 2012-2013 flu season, manufacturers have indicated shipments are likely to begin in August and continue throughout September and October until all vaccine is distributed. Doctors and nurses are encouraged to begin vaccinating their patients as soon as flu vaccine is available in their areas.

It takes up to two weeks to build immunity after a flu shot, but you can benefit from the vaccine even if you don't get it until flu season starts.

New flu vaccines are released every year to keep up with rapidly adapting flu viruses. Because flu viruses evolve so quickly, last year's vaccine may not protect you from this year's viruses.

After vaccination, your immune system produces antibodies that will protect you from the vaccine viruses. In general, though, antibody levels start to decline over time — another reason to get a flu shot every year.

The CDC recommends annual influenza vaccinations for everyone age 6 months or older. Vaccination is especially important for people at high risk of influenza complications, including:

• Pregnant women
• Older adults
• Young children

Chronic medical conditions can also increase your risk of influenza complications. Examples include:

• Asthma
• Cerebral palsy
• Chronic obstructive pulmonary disease (COPD)
• Cystic fibrosis
• Epilepsy
• HIV/AIDS
• Kidney or liver disease
• Muscular dystrophy
• Obesity
• Sickle cell disease

Check with your doctor before receiving a flu vaccine if:

• You're allergic to eggs. The flu vaccine contains tiny amounts of egg protein. If you have an egg allergy or sensitivity, you'll likely be able to receive a flu vaccine — but you might need to take special precautions, such as waiting in the doctor's office for at least 30 minutes after vaccination in case of a reaction.

• You had a severe reaction to a previous flu vaccine. The flu vaccine isn't recommended for anyone who had a severe reaction to a previous flu vaccine. Check with your doctor first, though. Some reactions might not be related to the vaccine.

The flu vaccine comes in two forms:

• A shot. A flu shot contains an inactivated vaccine made of killed virus. The injection is usually given in the arm. An intradermal (in the skin) vaccine is also available for people 18 to 64 years of age. Because the viruses in this vaccine are killed (inactivated), the shot won't cause you to get the flu, but it will enable your body to develop the antibodies necessary to ward off influenza viruses.
• A nasal spray. The nasal spray vaccine (FluMist) consists of a low dose of live, but weakened, flu viruses and is approved for use in healthy people 2 to 49 years of age who aren't pregnant. The vaccine doesn't cause the flu, but it does prompt an immune response in your nose and upper airways, as well as throughout your body.

Both the flu shot and the nasal spray help protect you from influenza. But there are differences to consider before deciding between the two.

No. The flu vaccine can't give you the flu. But you might develop flu-like symptoms — despite getting a flu shot — for a variety of reasons, including:

• Reaction to the vaccine. Some people experience muscle aches and fever for a day or two after receiving a flu shot. This may be a side effect of your body's production of protective antibodies. The nasal vaccine can cause runny nose, headache and sore throat.
• 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 circulating during the flu season. If this occurs, your flu shot won't protect you.
• Other illnesses. Many other diseases, such as the common cold, also produce flu-like symptoms. So you may think you have the flu when you actually don't.

Flu vaccines aren't 100 percent effective. According to the CDC, in past flu seasons when the match between flu vaccine and circulating strains of flu virus is close, a flu shot is between 60 and 70 percent effective in warding off influenza in all age groups combined.

With or without a flu shot, you can take steps to help protect yourself from the flu and other viruses. Good hygiene remains your primary defense against contagious illnesses.

• Wash your hands often and thoroughly with soap and water.
• Use an alcohol-based sanitizer on your hands if soap and water aren't available.
• Avoid touching your eyes, nose or mouth whenever possible.
• Avoid crowds when the flu is most prevalent in your area.

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