A live attenuated vaccine is a method of delivering weakened versions of a disease causing pathogen into the human body, where it can induce the immune system into preparing itself for any future encounters with the real, unadulterated virus.
This approach to vaccination has some considerable benefits, not the least of which is durable immunity against diseases that can withstand the test of time. Live vaccinations can usually immunise a person after a single dose, and regular boosters aren’t needed (as is the case with other types of vaccine). The MMR vaccine is an excellent example of an effective live vaccine that only requires one or two doses to achieve its effects. It is estimated that 90% of the people receiving the MMR injection will become immunised after the first dose, and a second dose is provided by the NHS childhood immunisation programme to account for the remaining 10% of the population.
The immune system is a very complex network of sub-systems activated that are activated by different pathogens, and live vaccines effectively activate all of the various elements of the immune response. This means that a more complete immune response occurs as a result of vaccination, and contributes to the fact that only one or two doses of live vaccination are actually needed to effectively immunise an individual.
Live vaccines achieve this end because they are as close to the live, wild-type virus as we can safely get in a vaccine. The results, as discussed above, are more efficient immunisations and a lessened need for booster injections.
Live, attenuated vaccines can also sometimes be delivered by alternative routes. For example a polio vaccine can be delivered orally, reducing the need for injections which many people might not be comfortable with.
Live attenuated vaccines can’t be safely given to people who are immunocompromised. Basically this means that these patients have weakened immune systems that struggle to deal with infections that a healthy person would barely register. This change in the immune system can come about as a consequence of disease (HIV damages the immune system) or certain kinds of treatment (like chemo- and radiotherapy for cancer). An immunocompromised person won’t be able to fend off attenuated bacteria and viruses, and would probably suffer serious illness rather than gain immunity.
These vaccines are also not given to women during pregnancies. The female body suffers a weakening of the immune system during the pregnancy because of the various pressures exerted by the developing foetus. Moreover, there is a chance of the vaccine making its way into the unborn child where it could cause serious damage.
Finally live, attenuated vaccines can sometimes pose logistical problems when it comes to transport and delivery. These vaccines are only effective if kept and stored at low temperatures, and it can prove difficult to maintain these conditions when a vaccine is being shipped off to a third world country for example.