Wednesday, 2 July 2008

S320: Book 6 - Modelling epidemics

This book looks at how epidemics happen, and what can be done to stop them.
It starts by looking at some very basic maths. It revolves around a number called R0, which is the average number of people who will be infected by a infective individual.
If the R0 is less than 1, then the infection will die out, if its greater it will survive.

It also looks at the affect of Herd Immunity. This has a big effect on diseases. Its brought on either by lots of the population having caught the disease and so being immune, or else through vaccination. The key thing here is that it is related to R0, if enough of the population is immune, then the R0 will drop below 1. This is because obviously most people an infected individual meets will be immune so will not be infected themselves. So with something like smallpox, you only need to get more than 85% of the population immune, and the disease will die out. This doesn't work so well with zoonotic diseases, as there is a reservoir of infections.

On the other hand, partial herd immunity can sometimes be a bad thing. If only a fraction of the population are immune, then the disease still thrives, but not as well. However the age at which one becomes infected tends to move up. This can be a problem, as some diseases caught in childhood (mumps for instance) can have much more severe consequences in adulthood.

Its a balance, and not always obvious what is going to be good in the long run!
An interesting book - and at only 70 odd pages, not too difficult to digest.

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