How does the vaccine work?
The flu vaccine is an inactivated viral vaccine. This means that the viruses predicted to be predominant during the current season are grown in chicken eggs, harvested, and then killed through chemical means. The vaccine is then purified and tested for safety. When taken by humans, the vaccine stimulates the production of antibodies that fight off the viruses included in the vaccine.

How effective is it?
Influenza vaccine effectiveness varies from year to year, depending on the degree of similarity between the virus strains included in the vaccine and those circulating during the flu season. The vaccine can prevent symptoms of flu infection in up to 90 percent of healthy young adults, but its effectiveness can be as low as 30 percent among very elderly, frail persons.

Who should get the flu vaccine?
Influenza shots are recommended for anyone who wishes to avoid the flu. It is specifically encouraged for people over the age of 65 years and all people who are at high risk for developing serious complications from the disease. These high-risk groups include children 6 months to 2 years and people with diabetes, immunosuppression, severe forms of anemia and chronic diseases of the heart, lung or kidneys, including asthma. The vaccine is also recommended for women who will be in the second or third trimester of pregnancy during flu season, health care personnel, residents of nursing homes or other chronic-care facilities, children and teenagers who are receiving long-term aspirin therapy, and people who are in close contact with someone in a high-risk group.

Who should not get the flu vaccine?
People who have had previous vaccine-associated allergic reactions should not get the vaccine. Also, people with egg allergies should avoid immunization because the viruses used in the vaccine are grown in hen's eggs. The vaccine may be given in the presence of minor illnesses, but adults with acute febrile illnesses should usually wait until their symptoms abate.

What are the side effects?
The flu vaccine causes no side effects in most people. Less than one-third of those who receive the vaccine have some soreness at the injection site, and about 5 to 10 percent experience mild side effects, such as headache or low-grade fever for about a day after vaccination. Rarely, an allergic reaction may occur among people who have severe allergies to eggs. Since the vaccine is not a live vaccine, it is not possible to get influenza from it.

How often should you get it?
The vaccine should be taken every year because flu viruses are constantly mutating and each year the vaccine is updated to include the most current flu strains. The vaccine should also be taken annually because antibodies produced by a person in response to the vaccine decline over time.

When is the best time to get vaccinated?
The best time to get vaccinated is September to mid-November. However, if you miss your flu shot in November it is still not too late to obtain one. It takes about two weeks after vaccination to develop antibodies against the flu. In the United States, the flu season occurs from about November until April but peak activity usually occurs between late December and early March.