Through natural genetic variability or induced mutation, a small percentage of the viral particles should have the capacity to infect the new host.
[11] This makes it easier for the host immune system to eliminate the agent and create the immunological memory cells which will likely protect the patient if they are infected with a similar version of the virus in "the wild".
[19] Vaccines function by encouraging the creation of immune cells, such as CD8+ and CD4+ T lymphocytes, or molecules, such as antibodies, that are specific to the pathogen.
[8] Live attenuated vaccines tend to help with the production of CD8+ cytotoxic T lymphocytes and T-dependent antibody responses.
Being able to produce a B-cell response as well as memory killer T cells is a key feature of attenuated virus vaccines that help induce a potent immunity.
[22] Additionally, within the five WHO-recommended live attenuated vaccines (tuberculosis, oral polio, measles, rotavirus, and yellow fever), severe adverse reactions are extremely rare.
[25] Some live attenuated vaccines have additional common, mild adverse effects due to their administration route.
[29] Pasteur applied this knowledge to develop an attenuated anthrax vaccine and demonstrating its effectiveness in a public experiment.
[31] The first rabies vaccine was subsequently produced by Pasteur and Emile Roux by growing the virus in rabbits and drying the affected nervous tissue.
[26] This technique was later used by several teams when developing the vaccine for yellow fever, first by Sellards and Laigret, and then by Theiler and Smith.
[29][32] The middle of the 20th century saw the work of many prominent virologists including Sabin, Hilleman, and Enders, and the introduction of several successful attenuated vaccines, such as those against polio, measles, mumps, and rubella.