Herd immunity refers to the total degree of immunity in a population. The eradication of an infectious disease is dependent on the establishment of a high degree of herd immunity against the pathogen. An epidemic of a disease is likely to occur when herd immunity against that disease is very limited, suggesting the presence of a greater number of susceptible people in the population.
Local immunity refers to immunity at a specific site, usually the site of pathogen invasion and multiplication. Secretory IgA antibodies in various body secretions provide local immunity. Plasma cells on mucosal surfaces or in secretory glands develop these antibodies locally. Natural infection or attenuated live virus vaccines administered orally or intranasally cause local immunity in the gut and nasal mucosas, respectively.