Attenuated virus

 For the bacterial use of the term, see Attenuator (genetics)

The verb "attenuate" means "to become weak."

Attenuation in virology is reducing the virulence of a virus, whilst keeping it viable (or 'live'), for the purpose of creating a vaccine. It is the counterpart of the vaccines produced by 'killing' the virus (inactivated vaccine)

Viruses may be attenuated via passage of the virus through a foreign host, such as :
 * tissue culture
 * embryonated eggs
 * live animals

The initial viral population is applied to the foreign host. In all likelihood one of these will possess a mutation that enables it to infect the new host. However this mutant will normally have a lower virulence in the original host, enabling it to infect them, but cause less damage, and so acts as a vaccine.

Advantages of Attenuated Vaccines

 * Activates all phases of the immune system (for instance IgA local antibodies are produced)
 * Provides more durable immunity; boosters are not required
 * Low cost
 * Quick immunity
 * Easy to transport/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)

Disadvantages

 * Major Disadvantage -Secondary mutation can cause a reversion to virulence
 * May still be able to cause disease in immunocompromised patients (e.g. those with AIDS)
 * Sometimes may not work in tropical areas