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Herd immunity will slow the rate of transmission of a disease within a population, with the magnitude of the effect depending on the level of herd immunity. If herd immunity is high, infection may fail to establish or can be eliminated from the population. It is not necessary for all individuals in a group to be immune to eliminate infection. The level of herd immunity (proportion of immune animals in the population) must simply be sustained at a level which exceeds a critical threshold value at which the contact rate between infectious and susceptible individuals is insufficient to sustain the epidemic. This means that if a minimum critical proportion of animals can be kept immune to infection, a disease can be eliminated from the population. For many infectious diseases, effective vaccination rates of 70-80% provide sufficient herd immunity to prevent an epidemic being sustained.