An attenuated vaccine is a vaccine created by reducing the virulence of a pathogen, but still keeping it viable (or "live"). 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).
Examples of "live" (example attenuated) vaccines include:
- Viral: measles vaccine, mumps vaccine, rubella vaccine, influenza vaccine, chicken pox vaccine, oral polio vaccine (Sabin), yellow fever vaccine, and nasal-spray flu vaccine (including the seasonal flu nasal spray and the 2009 H1N1 flu nasal spray). Rabies vaccines are now available in two different attenuated forms, one for use in humans, and one for animal usage.
- Bacterial: BCG vaccine, typhoid vaccine and epidemic typhus vaccine.
Viruses may be attenuated via passage of the virus through a foreign host, such as:
The initial microorganism population is applied to the 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 microorganisms to grow well in the new host; the result is a population that is significantly different to the initial population, and thus will not grow well in the original host when it is re-introduced to the original host (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".
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)
- 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)
- Initial "spotting" method first discovered by Easan Anand has very few side effects in comparison to injected antibody vaccines (e.g. tetanus)
Disadvantages of attenuated vaccines 
- Secondary mutation can cause a reversion to virulence.
- Can cause severe complications in immunocompromised patients.
- Can be difficult to transport due to requirement to maintain conditions (e.g. temperature)
- Badgett MR, Auer A, Carmichael LE, Parrish CR, Bull JJ (October 2002). "Evolutionary dynamics of viral attenuation". J. Virol. 76 (20): 10524–9. doi:10.1128/JVI.76.20.10524-10529.2002. PMC 136581. PMID 12239331.
- "Immunization". Archived from the original on 7 March 2009. Retrieved 2009-03-10.
- Levine MM, Ferreccio C, Black RE, Lagos R, San Martin O, Blackwelder WC (July 2007). "Ty21a live oral typhoid vaccine and prevention of paratyphoid fever caused by Salmonella enterica Serovar Paratyphi B". Clin. Infect. Dis. 45 Suppl 1: S24–8. doi:10.1086/518141?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%3dncbi.nlm.nih.gov. PMID 17582564.
- Shimizu H, Thorley B, Paladin FJ, et al. (December 2004). "Circulation of type 1 vaccine-derived poliovirus in the Philippines in 2001". J. Virol. 78 (24): 13512–21. doi:10.1128/JVI.78.24.13512-13521.2004. PMC 533948. PMID 15564462.
- Kroger, Andrew T.; Ciro V. Sumaya, Larry K. Pickering, and William L. Atkinson (2011-01-28). "General Recommendations on Immunization: Recommendations of the Advisory Committee on Immunization Practices (ACIP)". Morbidity and Mortality Weekly Report (MMWR) (Centers for Disease Control and Prevention). Retrieved 2011-03-11.
- 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