Herd immunity, or community immunity, is said to occur when the proportion of vaccinated people within a population (herd) provides protection to unprotected individuals. The theory is that in diseases passed from person to person, it is more difficult to maintain a chain of infection when large numbers are immune.
The higher the proportion of individuals who are immune, the lower the likelihood that a susceptible person will come into contact with an infected individual
Vaccination is thought to act as a sort of 'firebreak' in the spread of disease, slowing or preventing the further spread of the disease.
The proportion of immune individuals in a population above which a disease may no longer persist is the herd immunity threshold. Its value is said to vary according to the virulence of the disease and its method of spread from person to person. A few examples are:
Diphtheria: spread by saliva - threshold 85%
Pertussis (Whooping cough): spread by airborne droplets - threshold 92-94%
Smallpox: spread by social contact - threshold 83-85%
As of 2009, herd immunity to certain preventable diseases is said to be compromised by parental refusal: measles, whooping cough, mumps.
That is the theory of herd immunity and as with the theory of how vaccines work, I shall return to this topic in a later post.
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