Herd (population?) immunity occurs when a large number of individuals within a population are immunized against a particular disease. This results in a lower probablility of un-immunized individuals becoming infected. For example, susceptible elderly individuals are less likely to become infected with the flu if the people that care and visit them are vaccinated against flu. Have you heard of a case of smallpox, polio or diptheria lately? No, because those diseases are either non-existent or extremely rare today due to vaccination and herd immunity.
There is a good discussion of this concept in todays NYT by Arthur Allen, author of Vaccine: The Controversial Story of Medicine's Greatest Lifesaver.
The theoretical basis of the phenomenon of "herd immunity" can be expressed mathmatically thusly:
Ct+1 = St Ct r
Where C is the number of infected cases
S is the number of susceptible individuals
t is a given time period
t+1 is the next time period
r is a transmission parameter
The Basic Reproductive Rate (R0) is the mean number of secondary cases a typical infected indivdual will cause in an unimmunized population.
Still don't get it? Don't worry, go here to the NHS UK website and view the animation on herd immunity.
So if you love your elderly mom, dad, granny or gramps.....get vaccinated against flu.
Want to read the wingnut, scientifically illiterate views on vaccination go here or here.
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