In Social Networks
The standard mathematical definition of herd immunity applies only to "well-mixed populations," in which each infected individual is capable of transmitting the disease to any susceptible individual, regardless of social ties or location. More specifically, the relationship between the basic reproduction number R0 and the herd immunity threshold illustrated in the table above relies on a calculation that is valid only in well-mixed populations. Actual large populations, however, are better described as social networks, in which transmission can occur only between peers/neighbors. The shape of a social network can alter the level of vaccination required for herd immunity, as well as the likelihood that a population will achieve herd immunity. Compared to well-mixed populations, herd immunity in social networks is particularly fragile.
Read more about this topic: Herd Immunity
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