Centers for Disease Control and Prevention


Influenza Vaccine |
Why the Vaccine Must Be Taken Every Year |
When To Receive Influenza Vaccine |
Vaccine Information for the 1998-99 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 65 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, 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 are not vaccinated because of misperceptions about influenza and the vaccine. They mistakenly perceive influenza as merely a nuisance and believe that the vaccine causes unpleasant side effects or that it may even cause the flu. The truth is that influenza vaccine causes no side effects in most people. The most serious side effect that can occur after influenza vaccination is an allergic reaction in people who have severe allergy to eggs, since the viruses used in the vaccine are grown in hens' eggs. For this reason, people who have an allergy to eggs should not receive influenza vaccine.

Less than one-third of those who receive vaccine have some soreness at the vaccination site, and 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.

Nevertheless, some older people remember earlier influenza vaccines that did, in fact, produce more unpleasant side effects. Vaccines produced from the 1940s to the mid-1960s were not as highly purified as modern influenza vaccines, and it was these impurities that caused most of the side effects. Since the side effects associated with these early vaccines, such as fever, headache, muscle aches, and fatigue, were similar to some of the symptoms of influenza, people believed that the vaccine had caused them to get the flu. However, influenza vaccine produced in the United States has never been capable of causing influenza. The only type of influenza vaccine that has been licensed in the United States to the present time 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 do not receive influenza vaccine because they believe it is not very effective. There are several different 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.

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 produced by the host in response to the vaccine 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. Typically, activity is very low until December, and peak activity most often occurs between late December and early March. Influenza vaccine should be administered between September and mid-November. The optimal time for organized vaccination programs for persons at high risk for influenza-related medical complications is usually the period from October to mid-November. It takes about 1 to 2 weeks after vaccination for antibody against influenza to develop and provide protection.

Vaccine Information for the 1998-99 Influenza Season

The Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee (VRBPAC) has recommended that the trivalent influenza vaccine prepared for the 1998­99 season include A/Beijing/262/95-like (H1N1), A/Sydney/5/97-like (H3N2), and B/Beijing/ 184/93-like hemagglutinin antigens. For the B/Beijing/184/93-like antigen, U.S. manufacturers will use the antigenically equivalent strain B/Harbin/07/94 because of its growth properties. Although the current influenza vaccine can contain one or more of the antigens administered in previous years, annual vaccination with the current vaccine is necessary because immunity declines during the year following vaccination.

Recommendations for the Use of Influenza Vaccine

Influenza vaccine is strongly recommended for any person aged >6 months who-- because of age or underlying medical condition--is at increased risk for complications of influenza. Groups at increased risk of influenza complications include:
  • Persons aged >65 years
  • 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 children with 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 months­18 years) who are receiving long-term aspirin therapy and therefore might be at risk for developing Reye syndrome after influenza
  • Women who will be in the second or third trimester of pregnancy during the influenza season

Groups That Can Transmit Influenza to Persons at High Risk

Persons who are clinically or subclinically infected can transmit influenza virus to persons at high risk that they care for or live with. Some persons at high risk (e.g., the elderly, transplant recipients, and persons with AIDS) can have a low antibody response to influenza vaccine. Efforts to protect these members of high-risk groups against influenza might be improved by reducing the likelihood of influenza exposure from their caregivers. 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;
  • providers of home care to persons at high risk (e.g., visiting nurses and volunteer workers); and
  • household members (including children) of persons in high-risk groups.

Pregnant Women

Case reports and limited studies suggest that pregnancy may increase the risk for serious medical complications of influenza as a result of increases in heart rate, stroke volume, and oxygen consumption; decreases in lung capacity; and changes in immunologic function. Researchers estimated that an average of 1 to 2 hospitalizations among pregnant women could be prevented for every 1,000 pregnant women vaccinated. On the basis of these and other data that suggest that influenza infection may cause increased morbidity among women during the second and third trimesters of pregnancy, the Advisory Committee on Immunization Practices (ACIP) recommends that women who will be beyond the first trimester of pregnancy (>14 weeks' gestation) during the influenza season 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. Studies of influenza vaccination of more than 2,000 pregnant women have demonstrated no adverse fetal effects associated with influenza vaccine; however, more data are needed. Because currently available influenza vaccine is not a live-virus vaccine and major systemic reactions to it are rare, many experts consider influenza vaccination safe during any stage of pregnancy. However, because spontaneous abortion is common in the first trimester and unnecessary exposures have traditionally been avoided during this time, some experts prefer influenza vaccination during the second trimester to avoid coincidental association of the vaccine with early pregnancy loss.

Breastfeeding Mothers

Influenza vaccine does not affect the safety of breastfeeding for mothers or infants. Breastfeeding does not adversely affect immune response and is not a contraindication for vaccination.

Persons Traveling to Foreign Countries

The risk for exposure to influenza during travel to foreign countries varies, depending on season and destination. In the Tropics, influenza can occur throughout the year; in the Southern Hemisphere, most activity occurs from April through September. Because of the short incubation period for influenza, exposure to the virus during travel can result in clinical illness that begins while traveling, which is an inconvenience or potential danger, especially for persons at increased risk for complications. Persons preparing to travel to the Tropics at any time of year or to the Southern Hemisphere from April through September should review their influenza vaccination histories. If they were not vaccinated the previous fall or winter, they should consider influenza vaccination before travel. Persons in high-risk groups especially should be encouraged to receive the most current vaccine. Persons at high risk who received the previous season's vaccine before travel should be revaccinated in the fall or winter with the current vaccine.

General Population

Physicians should administer influenza vaccine to any person who wishes to reduce the likelihood of becoming ill with influenza. 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 (when vaccine for the upcoming influenza season becomes available), persons at high risk who are seen by health-care providers for routine care or as a result of hospitalization should be offered influenza vaccine. Opportunities to vaccinate persons at high risk for complications of influenza should not be missed. The optimal time for organized vaccination campaigns for persons in high-risk groups is usually the period from October through mid-November. It takes about 1 to 2 weeks after vaccination for antibody against influenza to develop and provide protection. In the United States, influenza activity generally peaks between late December and early March. High levels of influenza activity infrequently occur in the contiguous 48 states before December. Administering vaccine too far in advance of the influenza season should be avoided in facilities such as nursing homes, because antibody levels might begin to decline within a few months of vaccination. Vaccine should be offered to both children and adults up to and even after influenza virus activity is documented in a community.

Vaccination Administration Route

During recent decades, data on influenza vaccine immunogenicity and side effects have been obtained for intramuscularly administered vaccine. Because recent influenza vaccines have not been adequately evaluated when administered by other routes, the intramuscular route is recommended. Adults and older children should be vaccinated in the deltoid muscle and infants and young children 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, including pertussis vaccine (DTaP or DTP). Because influenza vaccine can cause fever when administered to young children, DTaP (which is less frequently associated with fever and other adverse events than is DTP) is preferable.

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), May 1, 1998 / Vol. 47 / No. RR-6, and "Update: Influenza Activity -- United States and Worldwide, 1997-98 Season, and Composition of the 1998-99 Influenza Vaccine," MMWR, April 17, 1998 / Vol. 47 / No. 14, p. 283. The MMWR is available at the following Internet address:

If you do not have access to Internet, you may call the toll-free number 888-CDC-FACTS (888-232-3228) to receive a hard copy of this report. Once the system has answered, you may bypass other disease information not specific to influenza by pressing options 2, 2, 1, 3, 4. 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.

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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