• People aged 65 or older
• People with certain chronic diseases
• Nursing home residents
• Children and teenagers on long-term aspirin therapy
• People who come in frequent contact with high-risk patients
• People with allergies to eggs should not receive the vaccine

• Influenza vaccine is the primary means of preventing influenza.
• The influenza vaccine is 70-90% effective in preventing illness in healthy young adults.
• Vaccine efficacy varies from year to year and depends on the match between the virus in the vaccine and the virus in circulation.

• The antivirals amantadine (Symmetrel) and rimantadine (Flumadine) can be used to prevent influenza A infection.
• These antivirals may prove beneficial in high-risk or at-risk persons who have not been vaccinated or in immunocompromised patients.

The prevention mainstay of influenza is the yearly vaccine. This vaccine can reduce the number of illnesses and deaths. The influenza vaccine is specifically recommended for people who are at high risk for developing serious complications as a result of flu infection, including:

• All people aged 65 years or older.
• People of any age with chronic diseases of the heart, lung or kidneys; diabetes; immunosuppression; or severe forms of anemia.
• Residents of nursing homes.
• Children and teenagers who are receiving long-term aspirin therapy and who may therefore be at risk for developing Reye syndrome.

Influenza vaccine is also recommended for those who are in close or frequent contact with other persons in the high-risk groups defined above, such as healthcare personnel and volunteers who work with high-risk patients. People who live in a household with a high-risk person should also consider the influenza vaccine.

It is accepted knowledge that influenza "infects" millions of people each year, causing complications that lead to over 150,000 hospitalizations and 20,000 deaths in the U.S. - all in the 4-month "flu season."

Vaccine efficacy varies from one person to another. Studies of healthy young adults have shown influenza vaccine to be 70-90% effective in preventing illness. For the elderly and those with certain chronic medical conditions, the vaccine is often less effective in preventing illness than in reducing the severity of illness and the risk of serious complications and death. Studies have shown the vaccine to reduce hospitalization by about 70% and death by about 85% among the elderly who are not in nursing homes. Among nursing home residents, vaccination can reduce the risk of hospitalization by about 50%, the risk of pneumonia by about 60%, and the risk of death by 75% to 80%.

Despite easy availability and access to the flu shot, only 55% of persons over age 65 receive it yearly. In those under age 65 but at high risk for complications, hospitalizations and possible death, the vaccination rate is less than 50%. Most people under age 65 and not at risk don't receive the vaccine, despite its positive benefits.

Some people fear its side effects, but the influenza vaccine causes no side effects in most people. The rare exception is a severe allergic reaction in people with an allergy to eggs. Since viruses used in the vaccine are grown in hens' eggs, those who are seriously allergic to eggs should not receive the influenza vaccine.

Less than one third of those who receive the vaccine have some soreness in the area where it was administered. About 5% to 10% experience mild side effects, such as headache or low-grade fever for about a day after vaccination. These side effects are most likely to occur in children who have not been exposed to influenza virus in the past.

Overall vaccine effectiveness varies from year to year because the strains included in the vaccine must be chosen 9 to 10 months before each influenza season. These predictions can miss a new strain. This happened in the 1997-1998 flu season with type "A/Sidney," a new strain that appeared after the vaccine composition was decided. The vaccine still had some effect by bestowing cross-immunity. Each year, the vaccine is updated to include the most current influenza virus strains. This is why it is necessary to get the flu shot every year.

Influenza usually occurs in the United States each year from October until April. Activity is typically low until December, and usually peaks between January and March. Influenza vaccine should be administered from mid-October to mid-November. It takes one to two weeks from the time of vaccination to develop antibodies and provide protection.

Some circumstances warrant the use of antivirals to prevent influenza. There are two antivirals used for prevention and treatment: amantadine (Symmetrel) and rimantadine (Flumadine). Both are very effective in preventing illnesses caused by type A influenza viruses. For maximum effectiveness, the drug must be taken each day for the duration of influenza activity in the community. The following groups may benefit from taking amantadine or rimantadine:

• High-risk persons who are vaccinated after the beginning of an influenza A outbreak in a community. These high-risk people can still be vaccinated after the outbreak; however, the development of antibodies following vaccination (in adults) can take up to 2 weeks. During this period, chemoprophylaxis should be considered.
• Persons providing care to those at high risk: household members, visiting nurses, volunteer workers, and unvaccinated employees of hospitals, clinics, and chronic-care facilities.
• Persons who cannot be vaccinated. Chemoprophylaxis may be considered for these persons during the period of peak influenza activity.
• Prophylaxis should be considered for all healthcare employees, regardless of their vaccination status, if the outbreak is caused by a variant strain of influenza A that might not be controlled by the vaccine.
• Immunocompromised patients (such as those with HIV). Chemoprophylaxis might be indicated for these high-risk persons who may have an inadequate antibody response to influenza vaccine.
• Persons who should not be vaccinated (e.g., those with severe anaphylactic hypersensitivity to egg protein or other vaccine components). Chemoprophylaxis throughout the influenza season or during peak influenza activity might be appropriate.