An attenuated vaccine is a vaccine created by reducing the virulence of a pathogen, but still keeping it viable. 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.
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. This process is known as "passage" in which the virus becomes so well adapted to the foreign host that it is no longer harmful to the vaccinated subject. 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
Live attenuated vaccines are administered via a viral transport media containing the relevant viral particles. The media is injected via a hypodermic needle or by inhalation with the method often dependent upon the source phage's virulence factors. A vaccine works by encouraging the creation of memory B and T cells specific for an antigen associated with the pathogen in question. Accordingly, a vaccine is only effective for as long as the body maintains a population of these cells. Non-Attenuated Vaccines typically require boosters to resubmit the antigen to these memory leukocytes, in a sense, this alerts the body that the pathogen is still a threat and that the population of specific memory leukocytes should be maintained rather than allowing them to die-off. This is much less of a concern for attenuated vaccines because the virus will reproduce, albeit at a severely reduced rate, resulting in constant antigen presence.
Advantages
Activates all phases of the immune system
Provides more durable immunity; boosters are required less frequently
Vaccines have strong beneficial non-specific effects. That is effects which go beyond the specific protective effects against the targeted diseases.
Disadvantages
In extremely rare cases, natural mutations can cause a reversion to virulence. In this case, the virus can revert to wild type or develop into an entirely new strain.
Live Vaccines are not usually recommended for immunocompromised patients due to the risk of potentially severe complications.
Live strains typically require advanced maintenance, such as refrigeration and fresh media, making transport to remote areas difficult and costly.
