Herd immunity
\hɜrd\ \ɪmˈjunəti\
Herd immunity is a form of protection from an infectious disease where enough people in a given population are immune to stop the continued spread of the disease.
Immunity from a disease can be achieved in a number of ways, such as vaccination or recovery from the disease itself. If enough people in a population are immune, the spread of the disease is reduced, which protects those who are not immune.
The percentage of people needed to be immune to the disease to achieve herd immunity depends on the reproduction number of the disease (R0), which itself depends on how contagious the disease is and the level of social distancing that occurs in the population. For example, measles has an R0 of between 12 and 18 and needs 92%-95% immunity to reach herd immunity.
If mitigation strategies such as social distancing and isolation are used to reduce a disease’s R0 then the level of herd immunity required will increase if the mitigation strategies are relaxed.