An attenuated virus is a strain of virus whose virulence has been lowered by physical or chemical processes, or by repeated passage through the cells of another species. Killed vaccines are generally used to provide immunization from organisms that are too virulent to be used in the living attenuated state. The immune system reacts to the presence of the pathogen in the same manner, whether the organism is live or dead. However, immunity produced by a live, attenuated vaccine, when possible, is usually more effective.
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