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Following a vaccination, a person can become immune to the specific disease. This immunity gives protection against illness in an individual.

Vaccinating the majority of the population against serious diseases can reduce the chance of people coming into contact with specific pathogens. This leads to herd immunity.

There are three recognised scenarios in relation to herd immunity:

1. • The majority of the population are not vaccinated against a specific disease but are healthy.
   • A few people are not vaccinated and ill and contagious.
   • This can develop easily into a mass infection because the majority of the population are not vaccinated.
2. • The majority of the population are not vaccinated against the specific disease but are well.
   • Some are vaccinated and healthy
   • A few are not vaccinated and ill and contagious.
   • Mass infection can result again, but a small number of vaccinated individuals remain healthy and some not vaccinated will also be healthy.
3. • The majority of the population are vaccinated against a specific disease and are healthy.
   • A few are not vaccinated but well.
   • A few are not vaccinated against the disease and they are ill and contagious.
   • The result is that the majority are protected due to the high level of vaccination. A few individuals will still become ill, but the large number of vaccinated individuals gives protection.

Immunisation against disease