Epidemics of influenza typically occur during the winter months in temperate regions and have been responsible for an average of approximately 36,000 deaths/year in the United States during 1990--1999 (1). Influenza viruses also can cause pandemics, during which rates of illness and death from influenza-related complications can increase worldwide. Influenza viruses cause disease among all age groups (2--4). Rates of infection are highest among children, but rates of serious illness and death are highest among persons aged > 65 years and persons of any age who have medical conditions that place them at increased risk for complications from influenza (2,5--7). Influenza vaccination is the primary method for preventing influenza and its severe complications.
What is influenza?
Influenza or "the flu" is a highly contagious viral infection of the nose, throat and lungs that is one of the most severe illnesses of the winter season. The flu is caused by the influenza virus. There are three types of influenza viruses: influenza A, B and C. Influenza A and B can cause serious disease and can lead to epidemics. The flu is spread easily from person to person, primarily when an infected person coughs or sneezes. It can lead to hospitalization or even death, especially among persons over the age of 65. Typically, the flu is characterized by the abrupt onset of high fever, chills, a dry cough, headache, runny nose, sore throat, and muscle and joint pain. It can cause extreme fatigue that may last days or weeks.
How effective are flu shots?
According to the CDC, flu shots have been shown to reduce hospitalization by approximately 70 percent and death by about 85 percent in free-living persons over age 65. Among nursing home residents, flu shots can reduce the risk of hospitalization by approximately 50 percent, the risk of pneumonia by about 60 percent and the risk of death by 75 to 80 percent.
What are the side effects of vaccination?
The most common type of side effect is soreness at the injection site. Soreness can last up to two days, is mild, and rarely will impair daily activities. Fever, malaise, muscle aches, and other symptoms can occur in persons with no previous exposure to the influenza virus used to make the vaccines, for example in children. Symptoms occur 6-12 hours following the injection and usually last one or two days.
Who should not get a flu shot?
Individuals with egg allergies or those who have had a previous vaccine-associated allergic reaction should avoid immunization. Persons with acute febrile illnesses (high fever) should usually wait until their symptoms subside. However, flu shots may be given in the presence of minor illnesses, with or without fever, particularly among children with mild upper respiratory tract infections or hay fever.
What strains does this year's flu vaccine include?
Each year a new flu vaccine is formulated to protect against new flu strains. The 2002-2003 influenza vaccine protects against A/Moscow/10/99 (H3N2)-like, A/New Caledonia/20/99 (H1N1)-like, and B/Hong Kong/330/2001-like strains.
Who should get influenza vaccine?
- People 65 years of age or older.
- Residents of nursing homes and other chronic-care facilities.
- Adults and adolescents with chronic pulmonary or cardiovascular disorders, including asthma.
- Health care workers, care givers and others who might transmit influenza virus to persons at high-risk for complications from infection.
- People who are less able to fight infections because of a disease they are born with, infection with Human Immunodeficiency Virus (HIV), treatment with medications such as long-term steroids, and/or treatment for cancer with X-rays or medications.
- Adults and adolescents who required regular medical follow-up or hospitalization during the preceding year because of chronic illnesses (including diabetes mellitus), kidney diseases, and blood cell diseases such as sickle cell anemia.
- Women who will be in the second or third trimester of pregnancy during the flu season (December - March).
- Persons 6 months to 18 years of age who receive long-term aspirin therapy and therefore might be at risk for developing Reye syndrome after influenza.
The influenza vaccine is very safe and effective and generally has few side effects. You cannot get influenza from the vaccine. There may be some soreness, redness or swelling at the injection site. Other possible mild side effects include a headache and low-grade fever for a day after vaccination. As with any medicine, there are very small risks that serious problems could occur after getting a vaccine. However, the potential risks associated with influenza illness are much greater than the potential risks associated with the influenza vaccine. * For the 2002-2003 flu season, high-risk individuals (e.g. the elderly, institutionalized, and people with chronic diseases) and health care workers should be immunized in October, all others should be immunized in November and December, or later.
FACT: Influenza can be prevented with a safe, effective vaccine.
Thimerosal, a mercury-containing compound, has been used as a preservative in vaccines since the 1930s and is used in multidose vials of inactivated influenza vaccine to reduce the likelihood of bacterial contamination. Although no scientific evidence indicates that thimerosal in vaccines leads to serious adverse events in vaccine recipients, in 1999, the U.S. Public Health Service and other organizations recommended that efforts be made to eliminate or reduce the thimerosal content in vaccines to decrease total mercury exposure, chiefly among infants (61--63).
The risks of severe illness from influenza infection are elevated among both young children and pregnant women, and both groups benefit from vaccination by preventing illness and death from influenza. In contrast, no scientifically conclusive evidence exists of harm from exposure to thimerosal preservative-containing vaccine, whereas evidence is accumulating of lack of any harm resulting from exposure to such vaccines (61,65). Therefore, the benefits of influenza vaccination outweigh the theoretical risk, if any, for thimerosal exposure through vaccination.
Because of the increased risk for influenza-related complications, women who will be pregnant during the influenza season should be vaccinated. Vaccination can occur in any trimester. One study of influenza vaccination of > 2,000 pregnant women demonstrated no adverse fetal effects associated with influenza vaccine (138).
Influenza vaccine does not affect the safety of mothers who are breastfeeding or their infants. Breastfeeding does not adversely affect the immune response and is not a contraindication for vaccination.
The risk for exposure to influenza during travel depends on the time of year and destination. In the tropics, influenza can occur throughout the year. In the temperate regions of the Southern Hemisphere, the majority of influenza activity occurs during April--September. 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 (e.g., on cruise ships) that include persons from areas of the world where influenza viruses are circulating (166,167). Persons at high risk for complications of influenza who were not vaccinated with influenza vaccine during the preceding fall or winter should consider receiving influenza vaccine before travel if they plan to
No information is available regarding the benefits of revaccinating persons before summer travel who were already vaccinated in the preceding fall. Persons at high risk who receive the previous season's vaccine before travel should be revaccinated with the current vaccine the following fall or winter. Persons aged >50 years and others at high risk should consult with their physicians before embarking on travel during the summer to discuss the symptoms and risks for influenza and the advisability of carrying antiviral medications for either prophylaxis or treatment of influenza.
- travel to the tropics,
- travel with organized tourist groups at any time of year, or
- travel to the Southern Hemisphere during April--September.
In addition to the groups for which annual influenza vaccination is recommended, 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 >6 months), depending on vaccine availability (see Influenza Vaccine Supply). 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.
Dosage recommendations vary according to age group. Among previously unvaccinated children aged < 9 years, 2 doses administered > 1 month apart are recommended for satisfactory antibody responses. If possible, the second dose should be administered before December. If a child aged < 9 years receiving vaccine for the first time does not receive a second dose of vaccine within the same season, only 1 dose of vaccine should be administered the following season. Two doses are not required at that time. Among adults, studies have indicated limited or no improvement in antibody response when a second dose is administered during the same season. Even when the current influenza vaccine contains one or more antigens administered in previous years, annual vaccination with the current vaccine is necessary because immunity declines during the year after vaccination. Vaccine prepared for a previous influenza season should not be administered to provide protection for the current season.
The intramuscular route is recommended for influenza vaccine. Adults and older children should be vaccinated in the deltoid muscle. A needle length > 1 inch can be considered for these age groups because needles < 1 inch might be of insufficient length to penetrate muscle tissue in certain adults and older children (173). Infants and young children should be vaccinated in the anterolateral aspect of the thigh (64). ACIP recommends a needle length of 7/8--1 inch for children aged < 12 months for intramuscular vaccination into the anterolateral thigh. When injecting into the deltoid muscle among children with adequate deltoid muscle mass, a needle length of 7/8--1.25 inches is recommended (64).
The CDC recommendations establish priority October vaccination for certain high-risk groups as indicated below. All other groups should seek vaccination in November and December.
Primary Changes and Updates in the Recommendations
The 2004 recommendations include four principal changes or updates:
- ACIP recommends that healthy children aged 6--23 months, and close contacts of children aged 0--23 months, be vaccinated against influenza (see Target Groups for Vaccination).
- Inactivated vaccine is preferred over live, attenuated influenza vaccine (LAIV) for vaccinating household members, health-care workers, and others who have close contact with severely immunosuppressed persons during periods when such persons require care in a protected environment. If a health-care worker receives LAIV, the health-care worker should refrain from contact with severely immunosuppressed patients for 7 days after vaccine receipt. No preference exists for inactivated vaccine use by health-care workers or other persons who have close contact with persons with lesser degrees of immunosuppression (see Live Attenuated Influenza Vaccine Recommendations/Close Contacts of Persons at High Risk for Complications from Influenza).
- Severely immunosuppressed persons should not administer LAIV. However, other persons at high risk for influenza complications may administer LAIV (see Personnel Who May Administer LAIV).
- The 2004--05 trivalent vaccine virus strains are A/Fujian/411/2002 (H3N2)-like, A/New Caledonia/20/99 (H1N1)-like, and B/Shanghai/361/2002-like antigens. For the A/Fujian/411/2002 (H3N2)-like antigen, manufacturers may use the antigenically equivalent A/Wyoming/3/2003 [H3N2] virus, and for the B/Shanghai/361/2002-like antigen, manufacturers may use the antigenically equivalent B/Jilin/20/2003 virus or B/Jiangsu/10/2003 virus (see Influenza Vaccine Composition).
- CDC and other agencies will assess the vaccine supply throughout the manufacturing period and will make recommendations in the summer preceding the 2004--05 influenza season regarding the need for tiered timing of vaccination of different risk groups.