An attenuated vaccine is a vaccine created by reducing the virulence of a pathogen, but still keeping it viable (or "live").^[1] Attenuation takes a living agent and alters it so that it becomes harmless or less virulent. These vaccines contrast to those produced by "killing" the virus (inactivated vaccine).

[edit] Examples

Examples of live vaccines include:

• Viral: polio vaccine (Sabin vaccine), measles vaccine, mumps vaccine, rubella vaccine, chicken pox vaccine, yellow fever vaccine^[2], and nasal-spray flu vaccine (including the seasonal flu nasal spray and the 2009 H1N1 flu nasal spray).

• Bacterial: BCG vaccine^[2], typhoid vaccine^[3]

[edit] Development

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

• Tissue culture
• Embryonated eggs
• Live animals

The initial microorganism 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. Eventually it will acquire many mutations to grow well in that host that it will be significantly different to the initial population. Thus when it is re-introduced to the original host, it doesn't grow as well (hence is "attenuated"). This makes it easier for the host's immune system to eliminate the agent and thus create the immunological memory cells which will likely protect the patient if they are infected with a similar version of the microorganism in "the wild".

[edit] Administration

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.

[edit] Advantages of attenuated vaccines

• 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
• 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)

[edit] Disadvantages of attenuated vaccines

• Secondary mutation can cause a reversion to virulence.^{[citation needed]}
• May still be able to cause disease, particularly in immunocompromised patients (e.g. those with AIDS)^{[citation needed]}
• Can be difficult to transport due to requirement to maintain conditions (i.e. temperature) at which the virus can "survive"

[edit] References

[edit] External links

• Global Polio Eradication Initiative: Advantages and Disadvantages of Vaccine Types
• CDC H1N1 Flu / 2009 H1N1 Nasal Spray Vaccine Q&A at the website of the US Centers for Disease Control and Prevention