Herd immunity
When a sufficiently high percentage of a population has been vaccinated and is immune to a given disease, that disease cannot spread. This also reduces the risk of infection for those who are not vaccinated. This is known as herd immunity.
Herd immunity varies between diseases. The more contagious the disease, the higher the vaccination coverage required for herd immunity. For example, the vaccination coverage for diphtheria needs to be at least 70 per cent, while the corresponding figure for measles is 95 per cent.
There are also diseases that cannot be eradicated by immunisation. The virus that causes tick-borne encephalitis lives mainly in non-human mammals and the continued existence of the disease cannot be changed by immunisation. Similarly, the tetanus bacteria lives in soil and infects people via cuts and broken skin. Herd immunity does not apply to these diseases because they are not infected from one person to another.