Vaccination introduces a weak or dead version of a pathogen to the body to bring about immunity. Public vaccination can bring herd immunity for some diseases.
If the number of people vaccinated against a specific disease drops in a population, it leaves the rest of the population at risk of mass infection, as they are more likely to come across people who are infected and contagious. This increases the number of infections, as well as the number of people who could die from a specific infectious disease.
Type of disease
It is important to note that herd immunisation does not work for all diseases. Herd immunity only works for diseases that are spread directly between people. For diseases that are not contagious - such as tetanus, which is contracted via bacteria in soil - herd immunisation is impossible.
Quality of vaccine
Herd immunity can also be affected by the quality and effectiveness of the vaccine being used. The geographic spread of the population is also a factor as herd immunisation can be less effective in centres of high population density where lots of people regularly come into daily contact.