Except for Dendritic cell vaccines, live vaccines consist of viruses or bacteria that replicate to a limited degree in the host in a manner resembling the natural microbe, thus eliciting an immune response like that elicited by natural infection.
Live vaccines are attenuated- their disease-causing capacity has been reduced by biological or technical manipulations. They need to be balanced by being neither overattenuated (no longer infectious to work as a vaccine) nor underattenuated (retaining even a limited amount of pathogenicity). This balance between incomplete attenuation (and resultant disease-causing ability) and complete attenuation (inadequately immunogenic) is delicate and technically unachievable for some microorganisms at present.
Such vaccines usually elicit both humoral (antibodies) and cellular immunity (e.g., cytotoxic T lymphocytes [CTLs]). They are not technically feasible for most vaccines under development.
As a live vaccine can replicate, it may be possible for it to revert to its more naturally pathogenic form unless there are several attenuating mutations.
Live attenuated vaccines can be rendered inactive by high ambient temperatures– a problem in tropical regions and settings with challenges in maintaining the cold chain.