Much of the illness and death caused by influenza can be prevented by annual influenza vaccination. Influenza vaccine is specifically recommended for people who are at high risk for developing serious complications as a result of influenza infection. These high-risk groups include:

• Those 65 years old or older
• Children 6 - 23 months old
• Adults and children with a chronic health condition
• Those who are more than 3 months pregnant during the flu season

The U.S. Advisory Committee on Immunization Practices (ACIP) has voted to prioritize vaccination efforts for 2002-2003 in October and earlier to target persons at high risk of complications from influenza, health care workers and children under 9 years of age who are receiving vaccine for the first time, because they need a booster dose one month after the initial dose. All other groups, including household members of high-risk persons, healthy persons 50-64, and others who wish to decrease their risk of influenza infection should begin vaccination in November. The optimal time to vaccinate is during October and November, although the change was made due to significant vaccine distribution delays during the 2000-01 and 2001-02 influenza seasons and the possibility of similar situations in future years.

In addition, the committee is "encouraging" influenza vaccination for healthy children aged 6 months to 23 months because children in this age group are at substantially increased risk for influenza-related hospitalizations. A full recommendation to annually vaccinate healthy children aged 6 months-23 months is expected to be made within the next one to three years.

The influenza recommendations for 2002 also encourage the use of influenza vaccine after November. Many people who should or want to receive influenza vaccine remain unvaccinated after November. Substantial amounts of vaccine have remained unused during the past two influenza seasons and extended vaccination efforts after November are needed to decrease illness and to ensure full use of vaccine supplies. Vaccine received after November is likely to be beneficial in most influenza seasons. Influenza activity has not peaked in the majority of recent seasons until late December through early March. Adults develop peak antibody protection against influenza infection 2 weeks after vaccination.

Vaccine manufacturers are projecting that a total of 88-93 million doses of influenza vaccine will be produced for the coming season. This is an early projection and may change as the season progresses. Influenza vaccine prepared for the 2002-03 season will include that same two A strains as those used in the past season. The B strain has not yet been selected.

Although annual influenza vaccination has long been recommended for people in the high risk groups, many still do not receive the vaccine. Some people do not receive influenza vaccine because they believe it is not very effective. There are several reasons for this belief. People who have received influenza vaccine may subsequently have an illness that is mistaken for influenza, and they believe that the vaccine failed to protect them. In other cases, people who have received vaccine may indeed have an influenza infection. Overall vaccine effectiveness varies from year to year, depending upon the degree of similarity between the influenza virus strains included in the vaccine and the strain or strains that circulate during the influenza season. Because the vaccine strains must be chosen 9 to 10 months before the influenza season, and because influenza viruses mutate over time, sometimes mutations occur in the circulating strains between the time vaccine strains are chosen and the next influenza season is over. These mutations sometimes reduce the ability of the vaccine-induced antibody to inhibit the newly mutated virus, thereby reducing vaccine efficacy.

Vaccine efficacy also varies from one person to another. Studies of healthy young adults have shown influenza vaccine to be 70% to 90% effective in preventing illness. In 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, vaccine 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%. When antigenic drift results in the circulating virus becoming different from the vaccine strain, overall efficacy may be reduced, especially in preventing illness, but the vaccine is still likely to lessen the severity of the illness and to prevent complications and death.

Some people are not vaccinated because of misconceptions about influenza and the vaccine. Many people are not aware of the seriousness of influenza infection and some believe that the vaccine can cause the flu. Influenza vaccine produced in the United States cannot cause influenza. The only type of influenza vaccine that has been licensed in the United States is made from killed influenza viruses, which cannot cause infection. An influenza vaccine that is made with live influenza viruses has been developed and may be marketed in the future. This vaccine is made with viruses that can confer immunity but do not cause classic influenza symptoms.

Some people worry about the side effects of influenza vaccine. While influenza vaccine, like any other vaccine or medicine, is capable of causing serious problems such as severe allergic reactions, the risk of the vaccine causing serious harm, or death, is extremely small. Almost all people who get influenza vaccine have no serious problems from it. The most common side effect from influenza vaccination is soreness at the site of the injection. The soreness can last up to 2 days but is usually mild and does not affect a person’s ability to perform their normal daily activities. Some people, usually children who have not been exposed to influenza virus in the past, may have fever and body aches after vaccination. These symptoms, if they occur, usually start 6-12 hours after vaccination and can continue for 1 or 2 days.

Less common side effects that can occur after vaccination include allergic reactions and Guillain-Barré syndrome (GBS), a severe paralytic illness. Life-threatening allergic reactions are very rare, but can happen in people who have severe allergy to any vaccine component, most commonly allergy to eggs. The influenza viruses used in the vaccine are grown in hens' eggs. People who have an allergy to eggs or who have ever had a serious allergic reaction to a previous dose of influenza vaccine should consult with a doctor before getting an influenza vaccination.

In 1976, swine flu vaccine was associated with an increased number of cases of GBS.   Influenza vaccines since then have not been clearly linked to GBS. However, if there is a risk of GBS from current influenza vaccines, it is estimated at 1 or 2 cases per million persons vaccinated – much less than the risk of severe influenza, which can be prevented by vaccination.

Although only a few different influenza viruses circulate at any given time, people continue to become ill with the flu throughout their lives. The reason for this continuing susceptibility is that influenza viruses are continually changing, usually as a result of mutations in the viral genes. Currently, there are three different influenza virus strains, and the vaccine contains viruses representing each strain. Each year the vaccine is updated to include the most current influenza virus strains. The fact that influenza viruses continually change is one of the reasons vaccine must be taken every year. Another reason is that antibody made after being vaccinated declines over time, and antibody levels are often low one year after vaccination.

In the United States, influenza usually occurs from about November until April, with activity peaking between late December and early March. The optimal time for vaccination of persons at high risk for influenza-related medical complications is usually the period from October to mid-November. However, to avoid missed opportunities for vaccination, vaccine should be offered to high-risk persons who are hospitalized or seen at their physician’s office starting in September and continuing through the winter. It takes about 1 to 2 weeks after vaccination for antibody against influenza to develop and provide protection.