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Concept
Attenuated vaccine
Summary
An attenuated vaccine (or a live attenuated vaccine, LAV) is a vaccine created by reducing the virulence of a pathogen, but still keeping it viable (or "live"). Attenuation takes an infectious agent and alters it so that it becomes harmless or less virulent. These vaccines contrast to those produced by "killing" the virus (inactivated vaccine).
Attenuated vaccines stimulate a strong and effective immune response that is long-lasting. In comparison to inactivated vaccines, attenuated vaccines produce a stronger and more durable immune response with a quick immunity onset. Attenuated vaccines function by encouraging the body to create antibodies and memory immune cells in response to the specific pathogen which the vaccine protects against. Common examples of live attenuated vaccines are measles, mumps, rubella, yellow fever, and some influenza vaccines.
Viruses may be attenuated using the principles of evolution via serial passage of the virus through a foreign host species, such as:
Tissue culture
Embryonated eggs (often chicken)
Live animals
The initial virus population is applied to a foreign host. Through natural genetic variability or induced mutation, a small percentage of the viral particles should have the capacity to infect the new host. These strains will continue to evolve within the new host and the virus will gradually lose its efficacy in the original host, due to lack of selection pressure. This process is known as "passage" in which the virus becomes so well adapted to the foreign host that it is no longer harmful to the subject that is to receive the vaccine. 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".
Viruses may also be attenuated via reverse genetics. Attenuation by genetics is also used in the production of oncolytic viruses.
Bacteria is typically attenuated by passage, similar to the method used in viruses.
Official source
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