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Immunization means giving all or part of a germ or a changed piece of the germ to cause the body to make an immune response that is just like the one acquired after natural infection. This usually presents little or no risk to the person getting the vaccine.
The idea is that the person getting an immunization ends up with complete lifelong protection against a disease or partial protection. Some vaccines must be given more than once over time to build or keep up the protection. The effectiveness of a vaccine is measured by how well it protects against the natural disease.
Killed or inactivated is the same thing when we talk about vaccines. A vaccine usually is made up of the infectious germ itself, either killed (inactivated) or "live-attenuated" (weakened). A person's immune status, age, and overall health must be taken into account before deciding which type of vaccine is given.
Killed vaccines contain dead viruses or parts of a virus that have been completely disabled and can no longer cause disease. Any viral part is inactivated in some way from causing harm to the person who receives the vaccine. Our immune systems are still able to build an immune response against the dead virus, so anyone can respond well if they come in contact and become infected with that virus at any time. Examples of these killed virus vaccines include the shots against polio, influenza, and hepatitis A.
Live-attenuated viral vaccines contain very weakened forms of the naturally occurring virus. These attenuated viruses can infect the body’s cells, which builds a much stronger immune response against the virus than the killed (or inactivated) types do. Being vaccinated with a live-attenuated form of a virus is just like being infected with the natural disease, but it is important to note that a live-attenuated virus will NOT cause the full disease.
Examples of vaccines with live-attenuated viruses are the MMR (measles, mumps, and rubella), the rotavirus vaccine given by mouth, and the influenza vaccine sprayed into the nose. Since the risk of side effects is higher for live attenuated vaccines, it is usually recommended that high-risk patients receive killed or inactivated vaccines.
Henry H. Bernstein, D.O., is a senior lecturer in Pediatrics at Harvard Medical School. In addition, he is chief of General Academic Pediatrics at Children's Hospital at Dartmouth and professor of pediatrics at Dartmouth Medical School. He is the former associate chief of General Pediatrics and director of Primary Care at Children's Hospital Boston. 
