An attenuated vaccine is a vaccine which uses live pathogenic material for the purposes of inducing immunity. The strength of the pathogen is weakened or attenuated during the processing of the vaccine to make it less likely to cause disease. A number of vaccines including typhoid, tuberculosis, polio, measles, mumps, and rubella vaccines may be offered in attenuated form.
Attenuation is usually accomplished by culturing the pathogen in a foreign host such as a tissue culture, a live animal, or an egg, with chicken eggs being especially popular. The cultured pathogen is introduced in low amounts to a patient, often through inhalation, where it multiplies in the body. Exposed to the pathogen in the form of an attenuated vaccine, the immune system responds and develops antibodies so that when someone encounters a full strength version, the body will be able to fight it off.
The idea behind an attenuated vaccine is that it is strong enough to cause immunity, but too weak to cause disease. However, the use of such vaccines has sometimes been linked with disease in some patients. In some cases this appears to be because a batch of vaccines was not handled properly, while in others, the patient reacted poorly, or the virus mutated inside the body to become virulent. This is why attenuated vaccines are viewed as more risky than killed or inactivated vaccines, in which the pathogen is killed before being introduced into the body so that it cannot make someone sick.
Given that attenuated vaccines can sometimes cause disease, one might be reasonably led to wonder why they are used at all when inactive vaccines are available and would presumably be much safer. One of the key reasons to use an attenuated vaccine is that they are more effective, with some vaccinations not even available in killed form. Live vaccines also stimulate a greater immune system response, lead to the development of more antibodies, and confer longer-lasting immunities. Additionally, they are less costly to produce than killed vaccines, making them appealing for rapid mass-vaccination efforts.
When someone is vaccinated with an attenuated vaccine, it is not uncommon to develop some minor symptoms of illness such as fever, fatigue, or sluggishness as the body's immune system reacts. A doctor can discuss common side effects of specific vaccines with patients and their families, and a doctor can also talk about more serious side effects which could occur. As a general rule, the personal and social benefit of vaccination is believed to outweigh the risks, but there are some specific circumstances in which it may be dangerous for someone to be vaccinated, making it critical to give a complete history to a doctor or nurse before receiving a vaccination or booster.