Flu Vaccine
(Influenza
Immunization)
Medical Revising Author: Melissa Conrad Stöppler, MD
Medical Revising Editor: Jay W. Marks, MD
What is flu?
The flu (or common flu) is a viral infection that
is spread from person to person in secretions of the nose and lungs, for example when sneezing.
Medically it is referred to as influenza. Flu is a respiratory infection, that is, an
infection that develops primarily in the lungs. Respiratory infections caused by
other viruses often are called flu, but this is incorrect. Influenza
usually causes higher fever, more malaise, and severe body aches. Although other
viruses may cause these symptoms, they do so less commonly.
The flu is a common
illness. Every year in the United States, on average:
- 5% to 20% of the population gets the flu,
- more than 200,000 people are hospitalized from flu complications, and
- about 36,000 people die from the flu.
Why vaccinate for the flu?
The flu is highly infectious and is a serious viral
respiratory infection.
Whereas with other viral respiratory infections the symptoms usually are mild
and most people can continue working or going to school while ill, with the flu,
the symptoms are severe and prolonged and cause individuals to miss days of work
or school. The infection stresses the body. In addition, superinfections may
occur. Superinfections are bacterial infections that occur on top of a
respiratory infection. Bacterial respiratory infections also are a serious type
of infection, and the simultaneous viral and bacterial infection can overwhelm
the function of the lungs and the body. Among the elderly and the very young, it can cause
death. Because of its infectiousness, morbidity (severity of symptoms and time
lost from work or school), and the potential for death, it is important to
prevent the flu by vaccination. Although there are medications to treat the flu,
they are expensive, not as effective as vaccination, and need to be started
within 24-48 hours of the start of symptoms.
What is the flu vaccine?
Flu vaccine is an inactivated vaccine, meaning that it
contains killed influenza virus. The killed influenza virus is injected into muscles and
stimulates the immune system to produce an immune response (antibodies) to the
influenza virus. When the virus enters a person who has been vaccinated, the
antibodies attack and kill the virus and prevent infection.
Each year, the
influenza virus can change slightly, making the vaccine used in previous years
ineffective. Each year, a new vaccine must be prepared that will be effective
against the expected type of influenza virus. The trick is to be able to predict
which influenza viruses are going to cause infection and to prepare a vaccine
against those viruses. Usually scientists can predict accurately which types of
influenza virus will cause infections and prepare an appropriate vaccine. The
viruses that are used to prepare flu vaccine are grown in eggs.
The vaccine is
generally effective against the influenza virus within two weeks of the
injection. The vaccine is only effective against the strains of the virus that
match the vaccine. These strains vary from flu season to flu season each year.
This is the reason that revaccination is required annually with the vaccine that
matches the strains of influenza that are currently prevalent.
Flu season can begin in October and last as late as May. October and November are considered the best times to receive the vaccination, but it is still effective when administered later.
Flu vaccination
does not protect against infection caused by microbes other than the influenza
virus.
Next: Who should receive the flu vaccine? »
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