Attenuated vaccine

An attenuated vaccine 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).

Examples of "live" (example attenuated) vaccines include:

Viruses may be attenuated via passage of the virus through a foreign host, such as:

The initial virus population is applied to a foreign host. One or more of these will possess a mutation that enables it to infect the new host. These mutations will spread, as the mutations allow the virus to grow well in the new host; the result is a population that is significantly different from the initial population, and thus will not grow well in the original host when it is re-introduced (hence is "attenuated"). 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 vaccinated subject. This makes it easier for the host's 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".

In an attenuated vaccine, live virus particles with very low virulence are administered. They will reproduce, but very slowly. Since they do reproduce and continue to present antigen beyond the initial vaccination, boosters are required less often. These vaccines are produced by growing the virus in tissue cultures that will select for less virulent strains, or by mutagenesis or targeted deletions in genes required for virulence. There is a small risk of reversion to virulence; this risk is smaller in vaccines with deletions. Attenuated vaccines also cannot be used by immunocompromised individuals.

In the near future, a developed live attenuated vaccine may also be used not only to prevent a disease, but for therapeutic purposes of an existing disease. If as for an example a patient has previously been infected by a "wild type" virus like the common herpes simplex virus, using a therapy approach in the form of herpes simplex live attenuated vaccine would achieve a desired result in such a manner that symptoms of the herpes simplex disease would be partially or fully cured. As a notable mention, a live herpes zoster vaccine for therapy is already available in the present time.

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