What is a vaccine?
A vaccine contains a part or a weakened form of a disease-causing virus or bacteria. It is given to a person who has not had that disease in order to prevent the person from getting sick in the first place. It does this by stimulating a response from the immune system. So, when a vaccinated person meets the actual disease, the immune system remembers the vaccine and is there to fight off the disease and keep the person from getting sick.
Vaccines are classified into two broad categories based on the type of antigen used: live-attenuated and inactivated.
Live-attenuated vaccines contain a weakened version of the virus or bacteria. When they go into the body, these weakened versions of the disease replicate, which then elicits an immune response as if the person was exposed to the disease without actually causing the disease. The virus is just strong enough to get the immune system to recognize it as the disease, but since it is not the disease itself, it cannot make you sick. It took ten years of growing the measles virus in a laboratory to transform it from the disease-causing virus to a version of it that is too weak to make a person sick. The immunity conferred from a live-attenuated vaccine is the same as the natural immunity a person gets from having the disease. Because the weakened form actually reproduces, it cannot be given to severely immunocompromised patients. For these individuals, their immune system is so fragile that they may not be able to turn a vaccination into immunity. Examples of live-attenuated vaccines include vaccines to protect against measles, mumps, rotavirus, rubella, and chickenpox (varicella).
Inactivated vaccines are not alive and cannot cause the disease they are meant to prevent, even in severely immunocompromised patients. They contain either whole cells that have been killed or parts of cells that have been separated but will still stimulate the immune system to mount a response against it. When administered, these vaccines are like the disease enough to generate an immune response, but since they are dead, they do not cause illness. Because they do not (and cannot) replicate in the body, more doses need to be given to develop an immune response strong enough to protect the person from the disease. Examples of inactivated vaccines include vaccines to protect against diphtheria, hepatitis A, hepatitis B, human papillomavirus, influenza, meningitis, pneumonia, tetanus and whooping cough (pertussis).