Can Attenuated Vaccines Replicate?

To produce an immune response, live, attenuated vaccines must replicate in the vaccinated person. A relatively small dose of administered virus or bacteria replicates in the body and creates enough of the organism to stimulate an immune response.

What is the major disadvantage of attenuated vaccines?

The major disadvantage of attenuated vaccines is that secondary mutations can lead to reversion to virulence and can thus cause disease. There is another possibility of interference by related viruses, as is suspected in the case of oral polio vaccine in developing countries.

Can attenuated vaccines become virulent?

The attenuated vaccine virus is excreted in the stool and may infect close contacts of the vaccinee, thus vaccinating them as well. However, because the vaccine virus is attenuated and undergoes replication in the gut, there is a possibility of reversion to a virulent form.