All of the following about 'herd immunity' are true, except:

A herd structure includes the population belong to the herd species, the presence and distribution of altered animal hosts and possible insect vector as well as those environmental and social factors that our or inhibit the spread of infection from host to host. This herd structure can never be constant as it is subjected to new variations. These new variations occur due to new bihs, new deaths and population mobility. Ref: Park's Textbook of Preventive and Social Medicine, 19th Edition; Pages 96 - 97