Herd immunity
Encyclopedia
Herd immunity describes 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. Herd immunity theory proposes that, in contagious disease
s that are transmitted from individual to individual, chains of infection
are likely to be disrupted when large numbers of a population are immune or less susceptible to the disease. The greater the proportion of individuals who are resistant, the smaller the probability that a susceptible individual will come into contact with an infectious individual.
Vaccination acts as a sort of firebreak
or firewall
in the spread of the disease
, slowing or preventing further transmission of the disease to others. Unvaccinated individuals are indirectly protected by vaccinated individuals, as the latter will not contract and transmit the disease between infected and susceptible individuals. Hence, a public health policy of herd immunity may be used to reduce spread of an illness and provide a level of protection to a vulnerable, unvaccinated subgroup. Since only a small fraction of the population (or herd) can be left unvaccinated for this method to be effective, it is considered best left for those who cannot safely receive vaccines because of a medical condition such as an immune disorder
or for organ transplant
recipients.
The proportion of immune individuals in a population above which a disease may no longer persist is the herd immunity threshold. Its value varies with the virulence of the disease, the efficacy of the vaccine, and the contact parameter for the population. No vaccine offers complete protection, but the spread of disease from person to person is much higher in those who remain unvaccinated.
It is the general aim of those involved in public health
to establish herd immunity in most populations. Complications arise when widespread vaccination is not possible or when vaccines are rejected by a part of the population. , herd immunity is compromised in some areas for some vaccine-preventable diseases, including pertussis
and measles and mumps
, in part because of parental refusal of vaccination.
Herd immunity only applies to diseases that are contagious. It does not apply to diseases such as tetanus
(which is infectious, but is not contagious), where the vaccine protects only the vaccinated person from disease. 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.
Immunity (medical)
Immunity is a biological term that describes a state of having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion. Immunity involves both specific and non-specific components. The non-specific components act either as barriers or as eliminators of wide...
that occurs when the vaccination
Vaccination
Vaccination is the administration of antigenic material to stimulate the immune system of an individual to develop adaptive immunity to a disease. Vaccines can prevent or ameliorate the effects of infection by many pathogens...
of a significant portion of a population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
(or herd) provides a measure of protection for individuals who have not developed immunity. Herd immunity theory proposes that, in contagious disease
Contagious disease
A contagious disease is a subset category of infectious diseases , which are easily transmitted by physical contact with the person suffering the disease, or by their secretions or objects touched by them....
s that are transmitted from individual to individual, chains of infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
are likely to be disrupted when large numbers of a population are immune or less susceptible to the disease. The greater the proportion of individuals who are resistant, the smaller the probability that a susceptible individual will come into contact with an infectious individual.
| Disease | Transmission | R0 | Herd immunity threshold |
|---|---|---|---|
| Diphtheria Diphtheria Diphtheria is an upper respiratory tract illness caused by Corynebacterium diphtheriae, a facultative anaerobic, Gram-positive bacterium. It is characterized by sore throat, low fever, and an adherent membrane on the tonsils, pharynx, and/or nasal cavity... |
Saliva | 6-7 | 85% |
| Measles Measles Measles, also known as rubeola or morbilli, is an infection of the respiratory system caused by a virus, specifically a paramyxovirus of the genus Morbillivirus. Morbilliviruses, like other paramyxoviruses, are enveloped, single-stranded, negative-sense RNA viruses... |
Airborne | 12-18 | 83 - 94% |
| Mumps Mumps Mumps is a viral disease of the human species, caused by the mumps virus. Before the development of vaccination and the introduction of a vaccine, it was a common childhood disease worldwide... |
Airborne droplet | 4-7 | 75 - 86% |
| Pertussis Pertussis Pertussis, also known as whooping cough , is a highly contagious bacterial disease caused by Bordetella pertussis. Symptoms are initially mild, and then develop into severe coughing fits, which produce the namesake high-pitched "whoop" sound in infected babies and children when they inhale air... |
Airborne droplet | 12-17 | 92 - 94% |
| Polio | Fecal-oral route | 5-7 | 80 - 86% |
| Rubella Rubella Rubella, commonly known as German measles, is a disease caused by the rubella virus. The name "rubella" is derived from the Latin, meaning little red. Rubella is also known as German measles because the disease was first described by German physicians in the mid-eighteenth century. This disease is... |
Airborne droplet | 5-7 | 80 - 85% |
| Smallpox Smallpox Smallpox was an infectious disease unique to humans, caused by either of two virus variants, Variola major and Variola minor. The disease is also known by the Latin names Variola or Variola vera, which is a derivative of the Latin varius, meaning "spotted", or varus, meaning "pimple"... |
Social contact | 6-7 | 83 - 85% |
| - R0 is the basic reproduction number Basic reproduction number In epidemiology, the basic reproduction number of an infection is the mean number of secondary cases caused by an individual infected soon after disease introduction into a population with no pre-existing immunity to the disease in the absence of interventions to control... , or the average number of secondary infectious cases that are produced by a single index case in completely susceptible population. |
|||
Vaccination acts as a sort of firebreak
Firebreak
A firebreak is a gap in vegetation or other combustible material that acts as a barrier to slow or stop the progress of a bushfire or wildfire. A firebreak may occur naturally where there is a lack of vegetation or "fuel", such as a river, lake or canyon...
or firewall
Firewall (construction)
A firewall is a fireproof barrier used to prevent the spread of fire between or through buildings, structures, electrical substation transformers, or within an aircraft or vehicle.- Applications :...
in the spread of the disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
, slowing or preventing further transmission of the disease to others. Unvaccinated individuals are indirectly protected by vaccinated individuals, as the latter will not contract and transmit the disease between infected and susceptible individuals. Hence, a public health policy of herd immunity may be used to reduce spread of an illness and provide a level of protection to a vulnerable, unvaccinated subgroup. Since only a small fraction of the population (or herd) can be left unvaccinated for this method to be effective, it is considered best left for those who cannot safely receive vaccines because of a medical condition such as an immune disorder
Immunodeficiency
Immunodeficiency is a state in which the immune system's ability to fight infectious disease is compromised or entirely absent. Immunodeficiency may also decrease cancer immunosurveillance. Most cases of immunodeficiency are acquired but some people are born with defects in their immune system,...
or for organ transplant
Organ transplant
Organ transplantation is the moving of an organ from one body to another or from a donor site on the patient's own body, for the purpose of replacing the recipient's damaged or absent organ. The emerging field of regenerative medicine is allowing scientists and engineers to create organs to be...
recipients.
The proportion of immune individuals in a population above which a disease may no longer persist is the herd immunity threshold. Its value varies with the virulence of the disease, the efficacy of the vaccine, and the contact parameter for the population. No vaccine offers complete protection, but the spread of disease from person to person is much higher in those who remain unvaccinated.
It is the general aim of those involved in public health
Public health
Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals" . It is concerned with threats to health based on population health...
to establish herd immunity in most populations. Complications arise when widespread vaccination is not possible or when vaccines are rejected by a part of the population. , herd immunity is compromised in some areas for some vaccine-preventable diseases, including pertussis
Pertussis
Pertussis, also known as whooping cough , is a highly contagious bacterial disease caused by Bordetella pertussis. Symptoms are initially mild, and then develop into severe coughing fits, which produce the namesake high-pitched "whoop" sound in infected babies and children when they inhale air...
and measles and mumps
MMR vaccine controversy
The MMR vaccine controversy was a case of scientific misconduct which triggered a health scare. It followed the publication in 1998 of a paper in the medical journal The Lancet which presented apparent evidence that autism spectrum disorders could be caused by the MMR vaccine, an immunization...
, in part because of parental refusal of vaccination.
Herd immunity only applies to diseases that are contagious. It does not apply to diseases such as tetanus
Tetanus
Tetanus is a medical condition characterized by a prolonged contraction of skeletal muscle fibers. The primary symptoms are caused by tetanospasmin, a neurotoxin produced by the Gram-positive, rod-shaped, obligate anaerobic bacterium Clostridium tetani...
(which is infectious, but is not contagious), where the vaccine protects only the vaccinated person from disease. Herd immunity should not be confused with contact immunity
Contact immunity
Contact immunity is the property of some vaccines, where a vaccinated individual can confer immunity upon unimmunized individuals through contact with bodily fluids or excrement...
, a related concept wherein a vaccinated individual can 'pass on' the vaccine to another individual through contact.
In social networks
The standard mathematical definition of herd immunity applies only to "well-mixed populations," in which each infected individual is capable of transmitting the disease to any susceptible individual, regardless of social ties or location. More specifically, the relationship between the basic reproduction number R0 and the herd immunity threshold illustrated in the table above relies on a calculation that is valid only in well-mixed populations. Actual large populations, however, are better described as social networks, in which transmission can occur only between peers/neighbors. The shape of a social network can alter the level of vaccination required for herd immunity, as well as the likelihood that a population will achieve herd immunity. Compared to well-mixed populations, herd immunity in social networks is particularly fragile.See also
- The mathematics of mass vaccination