What Is in the Flu Shot?


Vaccines are an artificial way to stimulate the body’s immune system into reacting to a foreign organism. After that artificial stimulus, the body is better prepared to deal with the real infection. Because vaccines need to be distributed over long distances from their place of manufacturing, they sometimes contain preservatives. Since vaccines introduce foreign organisms and chemicals into the body, some people develop side effects to the vaccines. However, the benefits from vaccines have been shown to outweigh the risks.

How Vaccines Work

When a person is exposed to a foreign organism, like a virus or a bacterium, the immune system mounts a response using specialized cells and proteins (antibodies). Depending on the infection, the immune system retains a “memory” of the invading organism. This is immunity. When an organism of a similar nature invades, the immune system is ready and reacts vigorously. Many times, people do not get a second case of the same disease because their immune system remembers how to fight the organism. Vaccines deliver a dead or attenuated (weakened) version of an organism, so the person receiving the infection develops immunity to it without becoming sick.

Flu Types and Strains

The flu comes in three types and many strains. Types A and B cause the most severe disease and outbreaks. Because of the genetic nature of the flu, the virus is prone to genetic mutations. Each time there is a significant mutation, a new strain of the flu develops. On average, these significant mutations, also called genetic shifts and drifts, occur once a year. As a result, influenza vaccines for different strains must be developed and given each year. Seasonal influenza vaccines contain dead or attenuated flu virus for two type A strains and one type B strain. In October 2009, an additional vaccine against H1N1 (“Swine”) flu, a type A virus, will also be offered in the many countries.


Vaccines are manufactured at large facilities in different countries. Because the entire world’s population is at risk of the flu and its complications, the vaccine must be delivered to some very remote areas. Like other products, the flu vaccine has protein in it (from the viruses) which bacteria may use to grow and multiply. To keep the bacteria from growing and ruining the vaccine while it is being transported, manufacturers add preservatives to the vaccine which keep bacteria from growing in it. The preservatives vary from manufacturer to manufacturer, but your health care provider should be able to tell you what preservatives are included in any vaccines you receive.

Side Effects

According to the Centers for Disease Control and Prevention, side effects from the flu shot include soreness, redness, or swelling at the site of the injection, a low grade fever, and/or aches. In a very low number of people who receive the vaccine, other side effects are seen. This is usually because these people have an egg allergy. Eggs are used to grow the flu virus before it is processed for the vaccine. In an even lower number of vaccine recipients (about 1 in 1 million), a severe disease of the nerves, called Guillain-Barre syndrome, is believed to occur. Because foreign viruses and substances are being injected into the body, some degree of reaction against them should be expected.


Since Dr. Jenner first developed the vaccine against smallpox, vaccines have saved many lives by protecting against very serious and life-threatening infections. It is estimated that about 36,000 people in the U.S. die from the flu each year. When enough people get their flu shot, all those who cannot get the shot because they have an allergy to the vaccine or because their immune systems are not healthy are protected as well. Given in time, the flu shot may keep you from some very severe complications, like pneumonia or death.

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