This article is a work in progress. Please feel free to contribute to it.
The multiplication factor (technically known as R, the "reproductive number") is important in a chain reaction - if each infected case infects two more, a runaway propagation ensues. 1.1 and it burns. 0.5 and it fizzles out. We aim to move through the possibly quite small range of conditions between the critical population and the sub-critical by isolation, sanitation and immunisation.
Immunisation for purposes of herd immunity presents an ethical problem. Not a large or difficult one, and one where the consensus answer is often in favour of doing it, but one that needs thought and which varies according to what immunisation is being considered and for what disease. As is common in problems in the real world, no available immunisation is purely for herd or social benefit, and probably none is purely for personal benefit. This complicates the calculation, beyond the powers of many who attempt it and instead adopt a simpler absolutism but also unfortunately beyond the powers of many who should be explaining it and working from an understanding rather than by rote.
Good conditions to consider include Rubella where the immunisation of boys is almost entirely for herd immunity.
Reproductive number (which gives a more detailed and mathematical explanation of herd immunity).
- Supercourse "Herd Immunity and Vaccination" online lecture
- Lecture "Concepts for the prevention and control of microbial threats – 2". Center for Infectious Disease Preparedness, UC Berkeley School of Public Health.