Attenuated virus

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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" [1]
  • Global Polo Eradication Initiative: Advantages and Disadvantages of Vaccine Types [2]