‘Herd immunity is a form of immunity that occurs when the vaccination of a significant portion of a population (or herd) provides a measure of protection for individuals who have not developed immunity.’

See how high vaccination rates protect the unvaccinated.

Unvaccinated group

When a contagious disease enters the unvaccinated group, many members are infected because they lack immunity against the disease. An important element of this is determined by the maturity and strength of the individual’s immune system. For example, every young child and elderly and frail adults might be more susceptible to infection because of immune system defects.

Successful Herd Community

When a large percentage of the population is vaccinated the spread of disease is limited. This indirectly protects unimmunized individuals, including those that cannot be vaccinated and those for whom vaccination was not successful.  This is the principle of Herd Community.

In diseases spread from person to person it is more difficult to maintain a chain of infection when much of population is vaccinated. As a numbers of those that are vaccinated are increases the protective effect of herd community increases. For some diseases herd community may begin to be induced with as little as 40% of population vaccinated. More commonly and depending of contagiousness of the disease, vaccination rates may need to be as high as 80-95%. This percentage is called herd community threshold.

Unsuccessful Herd Community

When only a small percentage of the population is vaccinated the risk of diseases out break is greater than if many are vaccinated. The unvaccinated members of population are not indirectly protected and each community member has a higher risk of becoming infected.

 

However, when immunization rates fall, herd immunity can break down leading to an increase in the number of new cases. For example, measles outbreak in the UK and pertussis outbreak in the US have been attributed to declining herd immunity.

“VaccinesToday” What is Herd  Immunity; March 16,2017, www.vaccinestoday.eu/stories/what-is-herd-immunity