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Why the Vaccine Must Be Taken Every Year |
When To Receive Influenza Vaccine |
Vaccine Information for the 2000-2001 Influenza Season

Influenza Vaccine

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 all people aged 50 years or older and people of any age with chronic diseases of the heart, lung or kidneys, diabetes, immunosuppression, or severe forms of anemia. Other groups for whom vaccine is specifically recommended are residents of nursing homes and other chronic-care facilities housing patients of any age with chronic medical conditions, women who will be more than 3 months pregnant during the influenza season, and children and teenagers who are receiving long-term aspirin therapy and who may therefore be at risk for developing Reye syndrome after an influenza virus infection. Influenza vaccine is also recommended for people who are in close or frequent contact with anyone in the high-risk groups defined above. These people include health care personnel and volunteers who work with high risk patients and people who live in a household with a high-risk person.

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.

Why the Vaccine Must Be Taken Every Year

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.

When To Receive Influenza Vaccine

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.

Vaccine for the 2000-2001 Influenza Season

The trivalent influenza vaccine prepared for the 2000-2001 season will include A/Moscow/10/99 (H3N2)-like, A/New Caledonia/20/99 (H1N1)-like, and B/Beijing/184/93-like antigens. For the A/Moscow/10/99 (H3N2)-like antigen, U.S. manufacturers will use the antigenically equivalent A/Panama/2007/99 (H3N2) virus and for the B/Beijing/184/93-like antigen, they will use the antigenically equivalent B/Yamanashi/166/98 virus; these viruses will be used because of their growth properties and because they are representative of currently circulating A (H3N2) and B viruses.

Recommendations for the Use of Influenza Vaccine
Influenza vaccine is strongly recommended for any person aged 6 months or older who -- because of age or underlying medical condition -- is at increased risk for complications of influenza. In addition, health-care workers and others (including household members) in close contact with persons in high-risk groups should be vaccinated to decrease the risk of transmitting infection to persons at high risk. Influenza vaccine also can be administered to any person who wishes to reduce the chance of becoming infected with influenza (the vaccine can be administered to children as young as 6 months).

Target Groups for Vaccination

Groups at High Risk for Influenza-Related Complications

Vaccination is recommended for the following groups of persons who are at increased risk for complications from influenza or who have a higher prevalence of chronic medical conditions that place them at risk for influenza-related complications:

  • persons aged 50 years or older;
  • residents of nursing homes and other chronic-care facilities that house persons of any age who have chronic medical conditions;
  • adults and children who have chronic disorders of the pulmonary or cardiovascular systems, including asthma;
  • adults and children who have required regular medical follow-up or hospitalization during the preceding year because of chronic metabolic diseases (including diabetes mellitus), renal dysfunction, hemoglobinopathies, or immunosuppression (including immunosuppression caused by medications);
  • children and teenagers (aged 6 months to 18 years) who are receiving long-term aspirin therapy and therefore might be at risk for developing Reye syndrome after influenza; and
  • women who will be in the second or third trimester of pregnancy during the influenza season.

Persons Who Can Transmit Influenza to Those at High Risk

Infected persons can transmit influenza virus to persons at high risk for complications from influenza. Efforts to protect members of high-risk groups against influenza might be improved by reducing the likelihood of influenza exposure from their care givers. Therefore, the following groups should be vaccinated:

  • physicians, nurses, and other personnel in both hospital and outpatient-care settings;
  • employees of nursing homes and chronic-care facilities who have contact with patients or residents;
  • employees of assisted living and other residences for persons in high-risk groups;
  • persons who provide home care to persons in high-risk groups; and
  • household members (including children) of persons in high-risk groups.

Pregnant Women

Women who will be beyond the first trimester of pregnancy (greater than or equal to 14 weeks' gestation) during the influenza season should be vaccinated.

Pregnant women who have medical conditions that increase their risk for complications from influenza should be vaccinated before the influenza season -- regardless of the stage of pregnancy. Because currently available influenza vaccine is an inactivated vaccine, many experts consider influenza vaccination safe during any stage of pregnancy.

Breast-feeding Mothers

Influenza vaccine does not affect the safety of mothers who are breast-feeding or their infants. Breast-feeding does not adversely affect immune response and is not a contraindication for vaccination.


The risk of exposure to influenza during travel depends on the time of year and destination. In the tropics, influenza can occur throughout the year, whereas most influenza activity occurs from April through September in the temperate regions of the Southern Hemisphere. In temperate climate zones of the Northern and Southern Hemispheres, travelers also can be exposed to influenza during the summer, especially when traveling as part of large organized tourist groups containing persons from areas of the world where influenza viruses are circulating.

Persons at high risk for complications of influenza should consider receiving influenza vaccine before travel if they were not vaccinated with influenza vaccine during the preceding fall or winter and they plan to a) travel to the tropics; b) travel with large organized tourist groups at any time of year; or c) travel to the Southern Hemisphere from April through September. Persons at high risk who received the previous season's vaccine before travel should be revaccinated with the current vaccine in the following fall or winter.

Because influenza vaccine might not be available during the summer in North America, persons aged 50 years or older and others at high risk might wish to consult with their physicians before embarking on travel during the summer to discuss the symptoms and risks of influenza and advisability of carrying antiviral medications for either prophylaxis or treatment for influenza.

General Population

Physicians should administer influenza vaccine to any person who wishes to reduce the likelihood of becoming ill with influenza (the vaccine can be administered to children as young as 6 months). Persons who provide essential community services should be considered for vaccination to minimize disruption of essential activities during influenza outbreaks. Students or other persons in institutional settings (e.g., those who reside in dormitories) should be encouraged to receive vaccine to minimize the disruption of routine activities during epidemics.

Timing of Influenza Vaccination Activities

Beginning each September, influenza vaccine should be offered to persons at high risk when they are seen by health-care providers for routine care or as a result of hospitalization. The optimal time to vaccinate persons in high-risk groups is usually from October through mid-November, because influenza activity in the United States generally peaks between late December and early March. Although vaccine generally becomes available in August or September, vaccine availability in any location cannot be assured consistently in the early fall. Therefore, persons planning large organized vaccination campaigns may consider scheduling these events after mid-October to reduce the possibility that the vaccination campaign will need to be canceled because vaccine is unavailable. Administering vaccine too far in advance of the influenza season should be avoided in facilities such as nursing homes, because antibody levels can begin to decline within a few months of vaccination. If regional influenza activity is expected to begin earlier than December, vaccination programs can be undertaken as soon as current vaccine is available. Vaccine should be offered to unvaccinated persons even after influenza virus activity is documented in a community.

Vaccination Administration Route

The intramuscular route is recommended for influenza vaccine. Adults and older children should be vaccinated in the deltoid muscle; a needle length of 1 inch or longer can be considered for these age groups. Infants and young children should be vaccinated in the anterolateral aspect of the thigh.

Simultaneous Administration of Other Vaccines, Including Childhood Vaccines

The target groups for influenza and pneumococcal vaccination overlap considerably. For persons at high risk who have not previously been vaccinated with pneumococcal vaccine, health-care providers should strongly consider administering pneumococcal and influenza vaccines concurrently. Both vaccines can be administered at the same time at different sites without increasing side effects. However, influenza vaccine is administered each year, whereas pneumococcal vaccine is not. Children at high risk for influenza-related complications can receive influenza vaccine at the same time they receive other routine vaccinations.

For additional information, see the "Prevention and Control of Influenza, Recommendations of the Advisory Committee on Immunization Practices (ACIP)," Morbidity and Mortality Weekly Report (MMWR), April 14, 2000/ Vol. 49 / No. RR-3, and "Update: Influenza Activity --- United States and Worldwide, 1999--2000 Season, and Composition of the 2000--01 Influenza Vaccine," MMWR, May 5, 2000 / Vol. 49 / No. 17, p. 375-381. The MMWR is available at the following Internet address:

If you do not have access to the Internet, you may call the toll-free number 888-CDC-FACT (888-232-3228) to receive a hard copy of the ACIP recommendations. Once the system has answered, you may bypass other disease information not specific to influenza by pressing options 2, 2, 1, 3, 4,3. At the prompt that asks if you want a current copy of the ACIP recommendations, please leave your name and address, and the document will be mailed to you.

The Influenza Vaccine Information Statement (VIS) is available at the following Internet address:

Persons needing further information regarding the use and availability of influenza vaccine should consult with their health-care provider or their local health department.

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Editor: Susan K. Boyer, RN
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