During the process of vaccination, an attenuated pathogen is inserted into the body the organisms. The attenuated pathogen is able to provoke the primary immune response in the body of the organism without causing any harm.
Once the primary immune response is generated, the body creates a memory in form of antibody to the pathogen. After vaccination, if the pathogen infects, the body recalls the memory and generates a secondary immune response. The intensity of the secondary immune response is much higher than the primary one, so, as soon as the pathogen enter in the body, the antibodies against it are activated and result in defeating the infection.
For a better understanding purpose, the intensity of the primary and the secondary immune response is given in the figure:
