2 Herd immunity and immunisation
This section introduces two important related concepts. Herd immunity describes the level of immunity that develops in a population, due to natural infection. Once a sufficiently large proportion of the population have become resistant, the disease can no longer spread because there are insufficient susceptible people and RTHerd immunity threshold’ (HIT) and it is different for each disease.
Critical immunisation threshold (qc) is the level of immunity that must be achieved in a population by vaccination, to stop a disease spreading. As you can see, it is conceptually very similar to the herd immunity threshold.