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Vaccine Study Center

About Vaccines

​​​Answers to frequently asked questions about vaccines

What are vaccines?

Vaccines are biological substances that stimulate the body’s immune system to prevent a specific disease.

How do they protect you?

Vaccines cause the body’s immune system to produce chemicals and antibodies, which are proteins that identify and neutralize harmful substances like bacteria and viruses. For example, the pneumococcal vaccine stimulates the immune system to produce the type of antibody that prevents infection from the organism, Streptococcus pneumonia, which can cause pneumonia.

Are there different types of vaccines?

Yes. Vaccines can be live and weakened ("attenuated"), or killed. Killed vaccines can use whole organisms or parts of organisms. In addition, there are now vaccines that use parts of organisms which have been produced in the laboratory using recombinant (genetic) technology.

What’s an adjuvant?

An adjuvant is a substance that increases the body’s immune response to an antigen — the part of an organism that stimulates the immune system. The antigen in some vaccines doesn’t stimulate the immune system very strongly. Manufacturers include adjuvants in some vaccines, particularly vaccines using small parts of organisms or those produced using recombinant (genetic) technology, so that the vaccines can trigger enough immune response to prevent disease. People with weakened immunity, whether from age, disease, or medications, may especially benefit from adjuvants. Live vaccines, in general, do not require adjuvants.

How safe are vaccines?

Because vaccines are generally given to healthy people, including infants, it is important that they be very, very safe. While we might tolerate a side effect such as drowsiness in a medicine given to patients who are sick — knowing that the medicine will help make them well — we expect only the highest level of safety from vaccines. There have always been worries about vaccines, because they are biological substances and they stimulate immunity, but vaccines accepted by the Food and Drug Administration (FDA) and administered to people in this country meet rigorous safety standards. The purpose of the VSC and of the Immunization Safety Office of the Centers for Disease Control and Prevention (CDC) is to ensure that our vaccine supply is safe.

When a disease is common, and everyone knows how bad it is, then most people are grateful to have a vaccine to prevent it. But once the vaccine is introduced and it becomes rare to see the disease anymore, people do not necessarily realize how harmful the disease is. Some may become concerned about getting a vaccine — perhaps it might cause a fever or a sore arm — or question the need for immunization, since the disease is not in evidence. It is important to continue giving vaccines and to maintain a high level of coverage in society, because, if we don’t, these diseases will come back, as occurred with measles in Great Britain in the 1990s.

What vaccines should my child or I get and when?

The Centers for Disease Control and Prevention (CDC) provides vaccine schedules for all ages.

How can I find more information about specific vaccines?

The Centers for Disease Control and Prevention (CDC) provides information about specific vaccines. See our list of resources for additional informative websites.​​