Herd immunity
Herd immunity (or
community immunity) describes a type of
immunity that occurs when the
vaccination of a portion of the
population (or herd) provides protection to unvaccinated individuals.
[1] Herd immunity theory proposes that, in diseases passed from person-to-person, it is more difficult to maintain a chain of infection when large numbers of a population are immune. The more immune individuals present in a population, the lower the likelihood that a susceptible person will come into contact with an infected individual.
[2]
The effectiveness of a
vaccine depends on the
immune system reaction which the patient develops.
[3] This involves the generation of long term memory
B cells and
T cells via
adaptive immunity following
innate immune responses. Sometimes the antigen contained in the vaccine doesn't trigger an immune response. In the latter case there is need for new and stronger vaccines.
Vaccination acts as a sort of "
firebreak" in the spread of the
disease, slowing or preventing further transmission of the disease to others.
[4] For example, if Person A had a disease and exposed Person B who was immune because of vaccination, Person B would not get ill and could not pass on the disease to Person C when he comes into contact with him. So even if Person C is not vaccinated, he indirectly gets protection from the disease.
[2] Hence herd immunity may be used to reduce spread of an illness and to protect a vulnerable, un-vaccinated subgroup. However because only a small fraction of the population (or herd) can be left un-vaccinated for this method to be effective, it is considered best left for those who cannot safely receive vaccines due a medical condition such as an
immune disorder or for
organ transplant recipients.
Although no vaccine offers 100% protection, the spread of disease from person to person is much higher in those who remain un-vaccinated.
[5] Virologists have found that when a certain percentage of a population is vaccinated, the spread of the disease is effectively stopped. This critical percentage, called the
herd immunity threshold, depends on the disease and the vaccine.
[4] It is the general aim of those involved in
public health to establish herd immunity in most populations. However complications arise when wide spread vaccination is not possible, and when vaccines fail (See the
MMR vaccine controversy in the
UK.) Additionally, some vaccines, such as the
tetanus vaccine, protect only vaccinated persons from
pathogens, and do not generate herd immunity.
[6]
Herd immunity should not be confused with
contact immunity, a related concept wherein a vaccinated individual can 'pass-on' the vaccine to another individual through contact.
cont'd...