Live, attenuated, vaccines contain a version of living microbes that has been weakened (attenuated) in a laboratory setting so they can no longer cause disease. Since they are very close to the actual infection, they elicit strong cellular and antibody responses. Live, attenuated vaccines often provide lifelong immunity with only one or two doses. Live, attenuated vaccines are usually more successful with viruses than bacteria.

Inactivated (killed) vaccines are generally better for use against bacteria. Scientists produce inactivated vaccines by killing the disease-causing microbe with chemicals, heat, or radiation. Most inactivated vaccines, however, stimulate a weaker immune system response than do live vaccines. Therefore, it may take several additional doses, called booster shots, to maintain a person’s immunity to a particular bacterium.