An attenuated vaccine involves the use of a live infectious agent whose virulence has been attenuated, so much so that it is no longer pathogenic for humans.
These vaccines are generally more powerful in inducing protective immunity than inactivated vaccines. In fact, an attenuated infectious agent is still able to replicate, even if within certain limits, in order to mimic a real infection.
The attenuated infectious agent is obtained by favoring its growth in cell lines (viruses) or culture media (bacteria), so as to reduce its growth capacity in human cells. The main problem with this type of vaccine is that the attenuation may not be stable, so the microorganisms could return to being virulent.
Vaccines for measles, mumps, poliomyelitis (Sabin's vaccine) and yellow fever consist of live and attenuated viruses.
