Attenuated vaccine
An attenuated vaccine is a vaccine created by reducing the virulence of a pathogen, but still keeping it viable (or "live").[1] 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).
Contents
Examples
Examples of "live" (example 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, oral polio vaccine (Sabin), rotavirus vaccine, and yellow fever vaccine.[2] Rabies vaccines are now available in two different attenuated forms, one for use in humans, and one for animal usage.
- Bacterial: BCG vaccine,[2] typhoid vaccine[3] and epidemic typhus vaccine.
Development
Viruses may be attenuated via passage of the virus through a foreign host, such as:
- Tissue culture
- Embryonated eggs
- Live animals
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 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".
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.
Advantages of attenuated vaccines
- Activates all phases of the immune system (for instance IgA local antibodies are produced)[citation needed]
- Provides more durable immunity; boosters are required less frequently[citation needed]
- Low cost[citation needed]
- Quick immunity[citation needed]
- 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)[citation needed]
- Vaccines have strong beneficial non-specific effects. That is effects which go beyond the specific protective effects against the targeted diseases.[4]
Disadvantages of attenuated vaccines
- Secondary mutation can cause a reversion to virulence.[5]
- Can cause severe complications in immunocompromised patients.[6]
- Can be difficult to transport due to requirement to maintain conditions (e.g. temperature)
References
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