Is it true that a vaccine can cause the disease it was meant to protect against?

The answer depends on the type of vaccine. Only vaccines made from live viruses or bacteria carry any risk of transmitting a disease, and even then, the risk is very small and the symptoms are generally very mild.

All vaccines create immunity by introducing a substance that triggers the body's immune system to react and produce antibodies, but not all vaccines are the same. Some are made with live viruses or bacteria that have been greatly weakened. Others are made with bacteria or viruses that have been killed. And still others use only specific parts of the bacteria or viruses to stimulate the body's defenses.

Here are the basic types of vaccines:

Attenuated vaccines are made from live viruses and live bacteria that have been weakened, usually by repeated replication in a lab. Because these organisms are alive, it's possible for them to cause a very mild form of the disease. This happens very rarely, but that's why doctors are cautious about giving live vaccines to anyone with a weakened immune system, such as someone being treated for cancer.

Only vaccines made from live viruses or bacteria carry even a small risk of transmitting the actual disease. The following vaccines on the U.S. Centers for Disease Control and Prevention (CDC) immunization schedule for children are made from live viruses:

• Chicken pox (varicella)
• MMR (measles-mumps-rubella)
• Rotavirus
• Flu – nasal spray only (flu shots do not contain live viruses)

None of the other vaccines on the immunization schedule, including the polio shot, are made from live viruses or bacteria. (The oral polio vaccine is made from live viruses but is no longer used in the United States.)

Inactivated or killed vaccines are made from bacteria or viruses that have been killed by heat or chemicals. You can't catch a disease from an inactivated vaccine because the infectious agent can't reproduce, but the dead virus or bacteria is still enough to stimulate the body's immune system. The flu shot and injected polio vaccine are inactivated vaccines.

Component or fractional or subunit vaccines are inactivated vaccines that are made from just a part of the virus or bacteria. For the Hib vaccine, for example, part of the coating of the bacteria is introduced. This stimulates immunity against the bacteria itself. The hepatitis A and hepatitis B vaccines and thepneumoccocal vaccine are component vaccines. Partial viruses and bacteria are unable to reproduce or cause disease.

Toxoid vaccines contain a toxin or chemical made by the virus or bacteria and so protect against the harmful effects of infection rather than the actual infection. These vaccines include the DtaP vaccine, which protects against diphtheria, tetanus, and pertussis. Toxoid vaccines do not contain the virus or bacteria and cannot cause disease.

Source from Babycenter.com