Influenza, commonly called "the flu," is an illness caused by
viruses that infect the respiratory tract. Compared with most other viral
respiratory infections, such as the common cold, influenza infection often
causes a more severe illness.
Causes of influenza:
The Influenza VirusesInfluenza viruses are divided into three types, designated A, B, and C.
Influenza types A and B are responsible for epidemics of respiratory illness
that occur almost every winter and are often associated with increased rates of
hospitalization and death. Influenza type C differs from types A and B in some
important ways. Type C infection usually causes either a very mild respiratory
illness or no symptoms at all; it does not cause epidemics and does not have
the severe public health impact that influenza types A and B do. Efforts to
control the impact of influenza are aimed at types A and B, and the remainder
of this discussion will be devoted only to these two types.
Influenza viruses continually change over time, usually by mutation. This
constant changing enables the virus to evade the immune system of its host, so
that people are susceptible to influenza virus infection throughout life. This
process works as follows: a person infected with influenza virus develops
antibody against that virus; as the virus changes, the "older" antibody no
longer recognizes the "newer" virus, and reinfection can occur. The older
antibody can, however, provide partial protection against reinfection.
Currently, three different influenza strains circulate worldwide: two type A
viruses and one type B. Type A viruses are divided into subtypes based on
differences in two viral proteins called the hemagglutinin (H) and the
neuraminidase (N). The current subtypes of influenza A are designated A(H2N1)
Influenza type A viruses undergo two kinds of changes. One is a series of
mutations that occur over time and cause a gradual evolution of the virus. This
is called antigenic "drift." The other kind of change is an abrupt change in
the hemagglutinin and/or the neuraminidase proteins. This is called antigenic
"shift." In this case, a new subtype of the virus suddenly emerges. Type A
viruses undergo both kinds of changes; influenza type B viruses change only by
the more gradual process of antigenic drift.
Symptoms of Influenza:
Typical clinical features of influenza include fever (usually 100° F to 103° F
in adults and often even higher in children) and respiratory symptoms, such as
cough, sore throat, runny or stuffy nose, as well as headache, muscle aches,
and often extreme fatigue. Although nausea, vomiting, and diarrhea can
sometimes accompany influenza infection, especially in children,
gastrointestinal symptoms are rarely prominent. The term "stomach flu" is a
misnomer that is sometimes used to describe gastrointestinal illnesses caused
by other microorganisms.
Most people who get the flu recover completely in 1 to 2 weeks, but some people
develop serious and potentially life-threatening medical complications, such as
pneumonia. In an average year, influenza is associated with about 20,000 deaths
nationwide and many more hospitalizations. Flu-related complications can occur
at any age; however, the elderly and people with chronic health problems are
much more likely to develop serious complications after influenza infection
than are younger, healthier people.
Much of the illness and death caused by influenza can be prevented by annual
influenza vaccination. Influenza vaccine is specifically recommended for those
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. Also, the
vaccine is not recommended while individuals have active infections or active
diseases of the nervous system.
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 up 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
Some people do not receive influenza vaccine because they believe it is not
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 Must The Vaccine 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.
should you receive the 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 January and 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. Who is at increased risk for complications of influenza by virtue
of age or underlying medical condition. Groups at increased risk of influenza
1. Persons aged 65 years or older.
2. Residents of nursing homes and other chronic care
facilities housing patients of any age who have chronic medical
3. Adults and children with chronic disorders of the
pulmonary or cardiovascular systems, including children with asthma.
4. 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,
5. Children and teenagers (6 months to 18 years) who
are receiving long-term aspirin therapy and therefore may be at risk for
developing Reye syndrome after influenza.
6. There is some evidence to suggest that women who are
in the third trimester of pregnancy or in the early postpartum
period may be at increased risk for serious medical complications after
influenza infection. Pregnant women who will be in the third trimester of
pregnancy between December and April should consult their health care
provider about receiving influenza vaccine during the period September to
In addition, the following groups should be vaccinated because, while not at
high risk themselves, they may transmit influenza to persons who are at high
risk for complications:
1. Physicians, nurses, and other health care personnel
in both hospital and outpatient-care settings.
2. Employees in nursing homes and chronic-care
facilities who have contact with patients or residents.
3. Providers of home care to persons at high risk--for
example, visiting nurses and volunteer workers.
4. Household members (including children) of high-risk
Finally, influenza vaccine may be administered to any person who wishes to
reduce his or her chances of acquiring influenza infection. Persons who provide
essential community services may be considered for vaccination to minimize
disruption of essential activities during influenza outbreaks. Students or
other persons in institutional settings, such as those who reside in
dormitories, should be encouraged to receive vaccine to minimize the disruption
of routine activities during epidemics. Persons needing further information
regarding the use and availability of influenza vaccine should consult with
their health provider or their local health department.
Antiviral Agents:Although vaccination is the primary method for control of influenza, antiviral
agents have a role in the prevention and treatment of influenza type A
infection. Amantadine and Rimantadine are chemically related drugs that
interfere with the replication cycle of influenza type A viruses. They are not
effective against influenza type B or other viral respiratory pathogens.
Safety of Amantadine and Rimantadine: Amantadine and Rimantadine can cause adverse reactions in some persons; such
adverse reactions are rarely severe, but may be important for some categories
of patients. Serious side effects, such as marked behavioral changes, delirium,
hallucinations, agitation, and seizures, have been observed. These more severe
side effects have been associated with high plasma drug concentrations and have
been observed most often in elderly persons, and persons with renal
insufficiency, seizure disorders, certain psychiatric disorders who have been
taking amantadine at a dose of 200 mg per day. Clinical observations and
studies have provided evidence that lowering the dosage of amantadine in these
persons will reduce the incidence and severity of such side effects;
recommendations for reduced dosages for these groups of patients have been
Modifications in dosage of amantadine or Rimantadine may be required for
persons with impaired renal or hepatic function, the elderly, children, and
persons with a history of seizures. The package insert should be reviewed
before use of amantadine or Rimantadine for any patient.
Amantadine- and Rimantadine-resistant influenza A viruses can emerge when either
of these drugs are administered for treatment; amantadine-resistant strains are
cross-resistant to Rimantadine. There is no evidence that amantadine- or
Rimantadine-resistant viruses are more virulent or more transmissible than
amantadine-or Rimantadine-sensitive viruses.
Resistant viruses have most frequently been isolated from persons taking one of
these drugs as therapy for influenza A infection. Resistant viruses have
sometimes been isolated from people who live in residential or institutional
settings where other residents are taking or have recently taken amantadine or
rimantadine as therapy. Persons with influenza-like illness should, to the
extent possible, avoid contact with uninfected persons, whether or not they are
taking amantadine or Rimantadine as treatment. It is advisable to discontinue
amantadine or Rimantadine treatment of persons who have influenza-like illness
as soon as clinically warranted, generally after 3-5 days of treatment or
within 24-48 hours after the disappearance of signs and symptoms.
Prevention: Both amantadine and Rimantadine are approximately 70% to 90%
effective in preventing illnesses caused by naturally occurring strains of type
A influenza viruses.
Treatment: In otherwise healthy adults, amantadine and Rimantadine have been
shown to reduce the severity and duration of signs and symptoms of influenza A
illness when administered within 48 hours of illness onset.
Influenza At A Glance:
Influenza, commonly called 'the flu', is caused by viruses that infect the
Influenza viruses are divided into three types, designated A, B, and C.
Most people who get the flu recover completely in 1 to 2 weeks, but some people
develop serious and potentially life- threatening medical complications, such
Much of the illness and death caused by influenza can be prevented by annual