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 :
Template:Cleanup-confusing 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)
- 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
- Badgett, MR. Oct 2002 Journal of Virology "Evolutionary dynamics of viral attenuation" 
- Global Polo Eradication Initiative: Advantages and Disadvantages of Vaccine Types