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Influenza ("The Flu") Facts
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Introduction
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. Top
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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. Top
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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. Top
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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. Top
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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. Top
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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. Top
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Who should get influenza vaccine?
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- 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.
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Vaccine Safety
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.
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Thimerosal
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. Top
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Pregnant Women
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). Top
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Breastfeeding Mothers
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. Top
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Travelers
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
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- travel to the tropics,
- travel with organized tourist groups at any time of year, or
- travel to the Southern Hemisphere during April--September.
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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. Top
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General Population
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. Top
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Dosage
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. Top
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Route
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). Top
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Recommendations
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:
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- 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.
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