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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 attenuated vaccines include:
- Viral: measles vaccine, mumps vaccine, rubella vaccine, live attenuated influenza vaccine (the seasonal flu nasal spray and the 2009 H1N1 flu nasal spray), chicken pox vaccine, smallpox vaccine, oral polio vaccine (Sabin), rotavirus vaccine, and yellow fever vaccine.
- Bacterial: BCG vaccine, oral typhoid vaccine and epidemic typhus vaccine.
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.
- Activates all phases of the immune system (for instance IgA local antibodies are produced)
- Provides more durable immunity; boosters are required less frequently
- Low cost 
- Quick immunity
- Some are easy to transport and administer (for instance OPV for polio can be taken orally, rather than requiring a sterile injection by a trained healthworker, as the inactivated form IPV does)
- Vaccines have strong beneficial non-specific effects. That is effects which go beyond the specific protective effects against the targeted diseases.
- Secondary mutation can cause a reversion to virulence. In this case, the virus can mutate back to the wild type virus or mutate further.
- Can cause severe complications in immunocompromised patients.
- Some can be difficult to transport due to requirement to maintain conditions (e.g. temperature)
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