Flu Vaccine
(Influenza Immunization or Flu Shot)

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Flu vaccine facts

  • There are two types of flu vaccines, the injection (with killed virus) and nasal spray vaccines (containing live, but weakened, virus).
  • Each year, the influenza virus can change slightly, making the vaccine used in previous years ineffective.
  • The vaccine is generally effective against the influenza virus within two weeks of administration.
  • The vaccine is only effective against the strains of the virus that match the vaccine.
  • The effectiveness of the flu vaccine is dependent upon the extent of the match between the virus strains used to prepare the vaccine and those viruses in actual circulation. The age and health status of the individual also play a role in determining the effectiveness of the vaccine.

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, the common flu 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 than other respiratory infections. Although other viruses may cause these symptoms, they do so less frequently.

Influenza viruses are divided scientifically 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. Influenza type C 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 of influenza types A and B. Type A viruses are divided into subtypes and are named based on differences in two viral surface proteins called hemagglutinin (H) and neuraminidase (N). There are 16 known H subtypes and nine known N subtypes.

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,
  • about 36,000 people die from the flu or its complications.

The swine flu pandemic of 2009 was caused by a novel influenza A virus designated H1N1 based upon its surface protein types. This virus was originally referred to as swine flu because many of the genes in this new virus were very similar to influenza viruses that normally occur in pigs in North America. However, this virus was actually quite different from the typical swine flu viruses found in pigs. The novel H1N1 virus first caused illness in Mexico and the United States in March and April 2009.

Medically Reviewed by a Doctor on 10/3/2013

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Flu Vaccination - Side Effects Question: Did you have any side effects of your flu vaccination?
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Flu Shot Side Effects

Many people worry about side effects from the flu shot, but serious complications are rare. Some people believe that they can actually get the flu from receiving the shot, but this is not the case. For the majority of people, the risks of developing the flu are far greater than any risks associated with the vaccine.


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