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January 7, 2000 (Updated January 12, 2000) What is influenza (flu)? Influenza, commonly called "the flu," is caused by the influenza virus, which
infects the respiratory tract. The virus is typically spread from person-to-person when an
infected person coughs or sneezes the virus into the air. Compared with other viral
respiratory infections such as the common cold, influenza infection can cause severe illness
and also precipitate serious and life-threatening complications in all age groups. Typical clinical features of flu include fever, dry cough, sore throat, runny or stuffy
nose, headache, muscle aches, and extreme fatigue. Children may experience gastrointestinal
symptoms such as nausea, vomiting, and diarrhea but such symptoms are not common in adults.
Although the term "stomach flu" is sometimes used to describe gastrointestinal
illnesses, this is caused by other microorganisms and is not related to true influenza. Some
of the medical complications precipitated by flu include bacterial pneumonia, dehydration, and
worsening of underlying chronic conditions, such as congestive heart failure and asthma.
Complications occur most often in persons who are particularly vulnerable, such as the elderly
and persons of any age with certain chronic health conditions. Is the United States undergoing a flu epidemic? In the United States, flu outbreaks typically occur during the winter months from late
December through March, leading to increased numbers of people with respiratory illness
("the flu"). The start, peak period, duration and total health impact (i.e.,
hospitalizations and deaths) of the flu season can vary considerably from year-to-year. CDC
monitors this activity and the associated influenza viruses and provides summaries of national
influenza surveillance results each week during October through April (with the exception of
the last week of the year during the holidays). These weekly summaries are available online http://www.cdc.gov/flu/weekly Do other respiratory viruses circulate during the influenza season? In addition to influenza viruses, it is important to note that other respiratory pathogens
also frequently circulate during the same time period as influenza viruses and can cause
similar respiratory illness. These pathogens include viruses such as respiratory syncytial
virus, which is the most common cause of severe respiratory illness in young children. Is there a new flu virus circulating this season? In the United States, the predominant influenza virus circulating this winter
(1999-2000) has been the influenza A/Sydney/5/97-like (H3N2) virus. This virus has been the
predominant influenza virus since the1997-98 flu season. Because influenza viruses change over
time through mutations (a process called "drift"), CDC works closely with
surveillance laboratories to detect emerging variant viruses. S o far this year, the 1999-2000
flu vaccine is well matched to the viruses circulating in the United States. Surveillance for
changes in circulating viruses continues year round. What can people do to protect themselves against flu? By far, the single most important preventive measure is for individuals, especially persons
at high risk for serious complications, to get vaccinated in the fall. Who should get a flu shot? The most important major group of persons who should receive flu vaccine are those
who are at high risk for developing serious complications from flu. These individuals include
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 influenza virus infection. The second major group of
persons who should be vaccinated against flu are those who are in close or frequent contact
with anyone in the high-risk groups defined above. These people include healthcare personnel
and volunteers who work with high-risk patients and people who live in a household with a
high-risk person. The primary reason for vaccinating this group is to decrease the risk of
passing influenza infection to persons at high risk for serious complications. Finally, anyone
who wishes to decreases their chances of developing flu can receive the flu vaccine. Is the flu vaccine effective immediately after a person receives the shot? It takes about 1 to 2 weeks after vaccination for antibody against influenza to develop and
provide protection. How well does the flu vaccine work? The effectiveness of flu vaccine in protecting individuals against illness or serious
complications of flu depends on primarily: 1) the age and health status of the person
receiving the vaccine and 2) the similarity or "match" between the virus strains in
the vaccine and those in circulation. This year's vaccine is well-matched to the virus
strains that have been circulating during the 1999-2000 flu season. When the
"match" between vaccine and circulating strains is close, flu vaccine prevents
illness in approximately 70%-90% of healthy persons younger than age 65 years. Among elderly
persons living outside chronic-care facilities (such as nursing homes), flu vaccine is 30%-70%
effective in preventing hospitalization for pneumonia and influenza. Among elderly nursing
home residents, flu vaccine is most effective in preventing severe illness, secondary
complications, and deaths. In this population, the vaccine can be 50%-60% effective in
preventing hospitalization or pneumonia and 80% effective in preventing death from influenza
infection. Should individuals still get vaccinated? The optimal time for organized vaccination campaigns to immunize persons in high-risk
groups against flu is from October through mid-November. However, provided that vaccine is
still available, flu vaccine should be administered to high-risk individuals who have not yet
been vaccinated even after mid-November. As long as flu vaccine is available, and flu activity
is continuing, it is appropriate for persons at high-risk for serious complications from
influenza to receive flu vaccine. How are the viruses for flu vaccine selected? Each year, influenza viruses are collected by many laboratories throughout the world,
including the United States. Some of these viruses are forwarded for further analysis to one
of four World Health Organization (WHO) reference laboratories, one of which is at the Centers
for Disease Control and Prevention in Atlanta. This information, along with epidemiologic
information on flu activity, is summarized and presented to an advisory committee of the Food
and Drug Administration and at a WHO meeting. These meetings result in the selection of three
viruses (two different subtypes of influenza A viruses and one influenza B virus) to go into
flu vaccine strains for the following fall and winter. Usually one or two virus strains change
in the vaccine each year. What are some of the most common misconceptions about flu? There are several common misconceptions about flu including the following: What are the adverse effects of flu vaccine? By far, the most common side effect of flu vaccine is local arm soreness and swelling .
This is usually mild and does not prevent most persons from working. The most serious side
effect that can occur after flu vaccination are uncommon and include severe allergic
reactions, particularly in people who have a severe allergy to eggs (the vaccine viruses are
grown in eggs). In addition, the 1976 swine flu vaccine was associated with Guillain Barre
Syndrome (GBS). Evidence for a causal relationship of GBS with subsequent vaccines prepared
from other virus strains is less clear. Further discussion can be found in "Prevention
and Control of Influenza: Recommendations of the Advisory Committee on Immunization Practices
(ACIP)." The document is available online at http://www.cdc.gov/epo/mmwr/preview/mmwrhtml/00057028.htm Why must flu vaccine be given every year? Influenza viruses are continually changing, which is why the viruses in the vaccine must be
updated often. Vaccine for one year may not cover viruses circulating in the next season.
Another reason is that antibody resulting from flu vaccine declines over time, and antibody
levels from one year can be too low to provide protection for the next year. Is there a shortage of flu vaccine this year? This year, over 80 million doses of flu vaccine were produced for distribution in the
United States . Typically, most of the flu vaccine supply is distributed and administered
during the fall and by late December and January, the amount of vaccine that is available is
limited. Individuals who want to be vaccinated should contact their regular health care
provider to inquire about the availability of flu vaccine. Healthcare providers can contact
either a local vaccine distributor or one of the manufacturers directly to inquire about the
availability of vaccine for purchase. If I am exposed to a person with flu and become infected, how soon will I get sick? The typical time from when a person is exposed to influenza virus to when symptoms begin is
about 1-4 days. How long is a person with flu contagious? This depends on the age of the person, but adults usually can shed influenza virus for up
to 3-5 days after they first develop symptoms. Some children may shed virus for longer than a
week. How many people get sick with the flu each year? How many die? Every flu season is unique but on average it is estimated that approximately 10% to 20% of
the general U.S. population develops flu, more than 110,000 persons are hospitalized from
flu-related complications, and approximately 20,000 Americans die from the complications of
flu. Will the new antivirals cure the flu? Four different antiviral drugs (amantadine, rimantadine, zanamivir and oseltamivir) have
been approved for treating influenza illness. All four drugs can reduce the duration of
moderate or severe symptoms of influenza by about one day if started within 48 hours of the
start of symptoms. The four drugs differ in terms of side effects. In some patients,
amantadine (Symmetrel®, others) can cause nervousness, difficulty concentrating, or
lightheadedness. Rimantadine (Flumadine®) can also cause similar types of side effects, but
less frequently. Caution is advised if zanamivir (Relenza®) is used by people who have asthma
or chronic obstructive pulmonary disease (COPD), because some people with asthma or COPD may
have bronchospasm or difficulty breathing after using zanamivir. Oseltamivir (Tamiflu®)can
cause nausea and vomiting in some people. All of these drugs are prescription drugs and a
physician should be consulted before the drugs are used. These drugs are effective against
influenza viruses, but they are not effective for illnesses caused by other viruses or
bacteria that can have symptoms similar to influenza. These drugs are also not effective for
treating bacterial infections that can occur as complications of influenza. Will antiviral medications prevent the flu? Two of the antiviral drugs (amantadine and rimantadine) have been approved for prophylaxis
(prevention) of influenza infection. These drugs are not, however, a substitute for influenza
vaccination. Both of these drugs are prescription drugs and a physician should be consulted
before the drugs are used for preventing influenza. How can a person tell if they have the flu, a cold, or some other respiratory
infection? It is not always possible to distinguish influenza illness from other viral or bacterial
respiratory illnesses on the basis of symptoms alone. A test must be performed to confirm that
an illness is influenza. In addition, a doctor’s evaluation may be needed to determine
whether a person has another infection complicating influenza. |
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CDC Home | Search | Health Topics A-Z This page last reviewed 1/12/2000 |