Herd Immunity

• Herd immunity occurs when a large percentage of a population is

  immunised. Therefore most of the population has been vaccinated

  against a disease and therefore the chances of a non-immune

  person coming into contact with an infected person is very low.

  Establishing herd immunity is important in reducing the spread of

  diseases.
• Non-immune people are protected as there is a lower probability

  of they will come into contact with infected individuals. Herd immunity provides protection for vulnerable subgroups of people who cannot be given the vaccination on medical grounds.
• The herd immunity threshold depends on:

  1.The disease

  2.The efficacy (effectiveness) of the vaccine

  3.The contact parameters for the population.
• For effective herd immunity, only a small number of people in a

  population can be left unvaccinated.
• The percentage of immune individuals in a population above which a

  disease no longer manages to persist is called the herd immunity

  threshold.

  Its value depends on the disease and is influenced by factors such as:

  • The type of disease.

  • The effectiveness of the vaccine.

  • The density of the population.
• Many countries have a public health policy for combating common diseases.

  In the UK for example mass vaccination begins in childhood resulting in herd immunity against diseases such as diphtheria, tetanus, poliomyelitis, influenza and whooping cough.
• Difficulties can arise when widespread vaccination is not possible due to poverty in the developing world, or when vaccines are rejected by a percentage of the population in the developing world.
• Sometimes adverse publicity about the vaccine can influence peoples’ choices as to whether to vaccinate their children or not. This has happened recently in the UK resulting in a measles outbreak. The level of herd immunity slipped below the threshold value because people believed that the vaccination could result in their child becoming autistic.
• Many pathogens have evolved mechanisms that evade the specific

  immune system which has consequences for vaccination strategies.

  Pathogens are able to change their genotype each time they

  reproduce by mutation of their DNA and through genetic

  recombination.

  This results in new strains continuously arising.
• The new strains of pathogen show antigenic variation as they

  have changed their surface antigens, the pathogen is now

  different to the original strain. This mean that the memory cells of

  the immune system are no longer effective against them.

  This happens in influenza. The continual production of new

  surface antigens enables the influenza virus to avoid the body’s

  immunological memory so that people need to be vaccinated

  each year with a new version of the vaccine. This is a major public

  health problem.