Live vaccines can be produced by attenuating the pathogen. Attenuation is normally done by serial passaging, where susceptible animal models or cell culture is infected with the pathogen before transferring the infection to a new subject. This is repeated until the virulence of the pathogen is reduced sufficiently for it to be administered to a patient.
Although these vaccines produce strong immune responses, it is possible for the virulence to return in a patient. This is known to happen with the live polio vaccine, with poliovirus being detected in waste water in London, UK recently.
As a live vaccine is able to replicate within the patient, it produces an immune response that is as close to a real infection as possible. Long-lasting memory cells are produced to give immunity that can last an entire lifetime from a single dose. Inactivated vaccines are unable to replicate, but can still produce a strong immune response. An adjuvant or booster dose may be required to ensure immunity is prolonged.