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Flu Vaccine

Flu Vaccine

Alternate Names

  • flu shot
  • influenza immunization
  • influenza vaccine


The flu vaccine involves the injection of inactivated viruses to protect against influenza, commonly known as the flu.

What is the information for this topic?

There are 3 types of influenza viruses: influenza A, B, and C.
Type C causes only mild disease in humans, does not occur in epidemics, and is not represented in the annual vaccine. Both types A and B mutate (change their genes slightly) and vary from year to year - type A more so than type B.
An injection of flu vaccine every year can provide protection for people who want to avoid the flu.
The flu vaccine contains three strains of killed flu virus - two type A strains and one for type B. It is produced roughly 9 to 12 months ahead of the flu season. The formula used is based on what experts believe will be the most common strains of flu virus in the coming season.
The vaccine has a 60% to 70% success rate in preventing influenza infection, though this varies by how well the vaccine strains match with the strains that actually wind up circulating during the season (usually October-March, peaking in January or February). The efficacy is higher against severe infections, hospitalizations, and deaths.
Because it takes about 2 weeks for a person to develop immunity following a flu shot, late September to mid-October is the best time to give the vaccine, though it can and should continue to be given to those who have not received it, even after influenza infections are being seen in the community.
The vaccine is recommended for people considered to be at high risk. These include:
  • people over age 50
  • children age 6-59 months
  • children and adolescents who are receiving long-term aspirin therapy and therefore might be at risk for Reye Syndrome if they contract influenza
  • children and adults with heart disease and lung disease, including asthma
  • people who live in nursing homes or other institutional settings
  • people who have a chronic disease such as diabetes, anemia, or kidney disease
  • people who can transmit the flu to others at high risk. This group includes healthcare workers and employees at facilities caring for people at high risk.
  • women who will be pregnant during the flu season
  • people whose immune system is weakened because of chemotherapy
  • people with immune deficiency diseases including HIV or AIDS
About 25% of adults who receive the flu vaccine report mild soreness at the site of the injection. Young children may develop fever after a flu vaccine.
In the 1970s, administration of a vaccine for swine flu was associated with Guillain-Barré syndrome, a rapidly progressive weakness that sometimes causes paralysis. Current influenza vaccines are being closely watched to see if this problem occurs again, but thus far studies have shown that if such a risk does exist, it is too small to verify.
There is a rare risk of an allergic reaction to the vaccine, with possible itching, trouble breathing, or shock. A person who has had this problem in the past should tell the person giving the shot about it. He or she may decide not to administer the vaccine in order to avoid the chance of a repeat reaction.
Individuals who should not get flu shots include the following:
  • people allergic to eggs
  • people with fever or illness more serious than a cold at the time of the shot
  • individuals who have had allergic reactions to the flu vaccine
  • people who have suffered paralysis from Guillain-Barré syndrome
A person who develops a high fever (a temperature above 100.5 degrees Fahrenheit or 38 degrees centigrade) or an allergic reaction after the shot should immediately call their healthcare provider. The provider can also discuss other questions or concerns about flu shots.
Live-attenuated influenza vaccine (LAIV) is given by inhalation through the nose. It consists of live but weakened flu viruses - the same three strains that are in the killed vaccine.
LAIV has been shown to be very effective and safe and is now recommended for persons age 5-49. Persons in this age range should receive LAIV rather than the injectable vaccine if possible, in order to spare short supplies of the injectable vaccine for young children and the elderly.

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