An inactivated vaccine (or killed vaccine) is a vaccine consisting of virus particles, bacteria, or other pathogens that have been grown in culture and then killed to destroy disease producing capacity. In contrast, live vaccines use pathogens that are still alive (but are almost always attenuated, that is, weakened). Pathogens for inactivated vaccines are grown under controlled conditions and are killed as a means to reduce infectivity and thus prevent infection from the vaccine. The virus is killed using a method such as heat or formaldehyde.
Inactivated vaccines are further classified depending on the method used to inactivate the virus. Whole virus vaccines use the entire virus particle, fully destroyed using heat, chemicals, or radiation. Split virus vaccines are produced by using a detergent to disrupt the virus. Subunit vaccines are produced by purifying out the antigens that best stimulate the immune system to mount a response to the virus, while removing other components necessary for the virus to replicate or survive or that can cause adverse reactions.
Because inactivated viruses tend to produce a weaker response by the immune system than live viruses, immunologic adjuvants and multiple "booster" injections may be required to provide an effective immune response against the pathogen. Attenuated vaccines are often preferable for generally healthy people because a single dose is often safe and very effective. However, some people cannot take attenuated vaccines because the pathogen poses too much risk for them (for example, elderly people or people with immunodeficiency). For those patients, an inactivated vaccine can provide protection.
The pathogen particles are destroyed and cannot divide, but the pathogens maintain some of their integrity to be recognized by the immune system and evoke an adaptive immune response. When manufactured correctly, the vaccine is not infectious, but improper inactivation can result in intact and infectious particles. Because the killed pathogens in a properly produced vaccine do not reproduce, booster shots are required periodically to reinforce the immune response.[inconsistent]
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