Herd immunity




Herd immunity refers to a form of indirect protection from infectious disease that occurs when a sufficient percentage of a population has become immune to an infection.



They may develop herd  immunity, through vaccination or previous infections, thereby reducing the likelihood of infection for individuals who lack immunity.


Disease spread occurs when some proportion of the population is susceptible to the disease.


Herd  immunity occurs when a significant portion of the population becomes immune to an infectious disease and the risk of spread from person to person decreases: those who are not immune are indirectly protected because ongoing disease spread is very small.



Immune individuals are unlikely to contribute to disease transmission, disrupting chains of infection, which stops or slows the spread of disease.



If herd immunity can be established and maintained in a population for a sufficient time, the disease is inevitably eliminated, with no more endemic transmissions occur.



If elimination is achieved worldwide and the number of cases is permanently reduced to zero, then a disease can be declared eradicated.



Eradication can thus be considered the final effect or end-result of public health initiatives to control the spread of infectious disease.



It is also known as indirect protection, community immunity, or community protection.



Refers to the protection of susceptible individuals against infection when is sufficiently large proportion of immune individuals exist in a population.



It is the inability of infected individuals to propagate an epidemic outbreak due to a lack of contact with sufficient numbers of susceptible individuals.



The greater the proportion of immune individuals in a community, the smaller the likelihood that non-immune individuals will come into contact with an infectious individual.



Eradication of smallpox and sustained reductions in disease incidence  in adults and those who are not vaccinated following routine childhood immunization with conjugated Hemophilus influenzae type B  and pneumococcal vaccines are successful examples of the effects of vaccine induced herd immunity.



The herd immunity threshold is considered as the proportion of individuals in a population who, having acquired immunity, can no longer participate in the chain of transmission.



The proportion of immune individuals in a population above this threshold will result in current outbreaks being extinguished and endemic transmission of the pathogen interrupted.



When  the herd immunity threshold has been reached, disease gradually disappears from a population.



Highly communicable pathogens, such as measles,  requires ab high proportion of the population to be immune to decrease sustained transmission.



For either naturally acquired or vaccine induced immunity the durability of the immune memory is a major determinant  to determine the population protection and sustaining herd immunity.



Long-term immunity can occur with measles, varicella, and rubella, with infection as well as vaccination. 



With infections associated with transient immunity, the number of susceptible individuals soon increases in the absence of vaccine and outbreaks will reappear.



An effective vaccine program can sustain herd immunity and outbreaks can be curtailed as long as the community maintains the necessary levels.



Herd immunity thresholds assuming random mixing between individuals in a population, however some individuals have a higher number of interactions than others. 



Individuals that have higher numbers of interactions get infected earlier in outbreaks.



There is no evidence for a large scale successful intentional infection-based herd immunity strategy.



Some individuals cannot become immune because of medical conditions: immunodeficiency or immunosuppression, and for this group herd immunity is a crucial method of protection.



When herd immunity is achieved worldwide, it may result in the permanent reduction in the number of infections to zero, called eradication.



Herd immunity applies only to contagious disease, transmitted from one individual to another.



Opposition to vaccination has posed a challenge to herd immunity, allowing preventable diseases to persist in or return to populations with inadequate vaccination rates.



Herd immunity threshold (HIT) varies depending on the infectious disease. 



Influenza, has a herd immunity threshold of 33-44%. 



Measles a high threshold with a HIT of 92-95%.



Newborn infants are too young to receive many vaccines, either for safety reasons or because passive immunity renders the vaccine ineffective.



Individuals who are immunodeficient may have lost any immunity that they previously had and vaccines may not be of any use.



High levels of immunity in one age group can create herd immunity for other age groups: pertussis vaccine in adults reduces pertussis incidence in infants too young to be vaccinated, who are at the greatest risk of complications from the disease; children receiving vaccines against pneumococcus reduces pneumococcal disease incidence among younger, unvaccinated siblings.



Vaccinating children against pneumococcus and rotavirus has had the effect of reducing pneumococcus- and rotavirus-attributable hospitalizations for older children and adults, who do not normally receive these vaccines.



Influenza is more severe in the elderly than in younger age groups, but influenza vaccines lack effectiveness in this older demographic due to a waning of the immune system with age.



Prioritizing school-age children for seasonal flu immunization, is more effective than vaccinating the elderly, as it has been shown to create a certain degree of protection for the elderly.



For sexually transmitted infections high levels of immunity in one sex induces herd immunity for both sexes.



Vaccines against STIs targeting one sex,  result in significant declines in STIs in both sexes if vaccine uptake in the target sex is high.



Herd immunity itself acts as an evolutionary pressure on certain viruses, influencing viral evolution by encouraging the production of novel strains.



This is referred to as escape mutants, which  are able to escape from herd immunity and spread more easily.



Viruses escape from herd immunity through antigenic drift.



Antigenic drift occurs is when mutations accumulate in the portion of the viral genome that encodes for the virus’s surface antigen.



The virus�s surface antigen is 


typically a protein of the virus capsid, producing a change in the viral epitope.



The reassortment of separate viral genome segments, and antigenic shift, can also produce new serotypes.



With such changes memory T cells no longer recognize the virus, so people are not immune to the dominant circulating strain.



With nfluenza and norovirus epidemics, temporarily induced herd immunity occurs until  a new dominant strain emerges, causing successive waves of epidemics.



Similarly, the initial vaccines against Streptococcus pneumoniae reduced nasopharyngeal carriage of vaccine serotypes, only to be offset by increased carriage of non-vaccine serotypes: subsequent pneumococcal vaccines that provide protection from the emerging serotypes.



R0 refers to the estimated infectivity of a disease.



Herd immunity threshold is the percentage of the population that has immunity to the disease.



The critical value, or threshold, in a given population, occurs when the disease reaches an endemic steady state, which means that the infection level is neither growing nor declining exponentially. 



Estimated R0 and HITs (herd immunity threshold) of  infectious diseases: 



Measles Airborne 12-18 92-95%



Pertussis Airborne droplet 12-17 92-94%



Diphtheria Saliva 6-7 83-86%



Smallpox 5-7 80-86%



Mumps Airborne droplet 4-7 75-86%



COVID-19 2.5-5.7  60-75%





(2002-2004  2-5 50-80%






Bodily fluids 1.5-2.5 33-60%






Influenza pandemics-Airborne droplet 1.5-1.8 33-44%



R0 functions as a measure of contagiousness, so low R0 values are associated with lower HITs, whereas higher R0s result in higher HITs: 


the HIT for a disease with an R0 of 2 is theoretically only 50%, whereas a disease with an R0 of 10 the theoretical HIT is 90%.



The effective reproduction number, Re, is the number of people in a population who can be infected by an individual at any specific time. 



The Re number changes as the population becomes increasingly immunized, either by individual immunity following infection or by vaccination, and also as people die.



When the reproduction number (Re) of a contagious disease is reduced to below 1 new individual per infection, the number of cases occurring in the population gradually decreases until the disease has been eliminated.



When the population of individuals immune to a disease exceeds that disease’s HIT, the number of cases reduces at a faster rate, outbreaks are less likely to happen, and outbreaks are smaller than they would be otherwise.



When,the effective reproduction number increases to above 1, then the disease is neither in a steady state nor decreasing in incidence, but is actively spreading through the population and infecting a larger number of people than usual.



Theoretically above calculations assume that populations are homogeneous, or that every individual comes into contact with every other individual, when in reality populations are better described as social networks.



In social networks individuals tend to cluster together, with relatively close contact with a limited number of other individuals. 



In social networks, transmission only occurs between those who are geographically or physically close to one another.



A networks shape and size alter a disease’s HIT.



The cumulative proportion of individuals who get infected during the course of a disease outbreak can exceed the HIT. 



The herd immunity threshold does not represent the point at which the disease stops spreading.



The herd immunity threshold is 


the point at which each infected person infects fewer than one additional person on average. 



When the HIT is reached, the number of additional infections begins to taper off.



The term overshot refers to the difference between the cumulative proportion of infected individuals and the theoretical HIT.



The primary way to boost levels of immunity in a population is through vaccination.



Vaccines provide protection in a far safer way than natural infections: they generally do not cause the diseases they protect against and severe adverse effects are significantly less common than complications from natural infections.



The immune system does not distinguish between natural infections and vaccines.



The immune system actively  responds  to natural infections and vaccines.



Thebimmunity induced via vaccination is similar to what would have occurred from contracting and recovering from the disease.



After the successful introduction and widespread use of a vaccine, sharp declines in the incidence of diseases it protects against can be observed..



There are decreases the number of hospitalizations and deaths caused by such diseases.



Waning vaccine-induced immunity, as occurs with acellular pertussis vaccines, requires higher levels of booster vaccination to sustain herd immunity.



When a disease has ceased to be endemic to a population, natural infections no longer contribute to a reduction in the fraction of the population that is susceptible: Only vaccination contributes to this reduction.



Passive immunity can also be gained when antibodies to a pathogen are transferred from one individual to another: maternal antibodies, primarily immunoglobulin G antibodies, are transferred across the placenta and in colostrum to fetuses and newborns.



Passive immunity can also be gained when a susceptible person is injected with antibodies from the serum or plasma of an immune person.



Protection obtained  by passive immunity is immediate, but wanes over the course of weeks to months, and its contribution to herd immunity is temporary.



For diseases among fetuses and newborns, such as influenza and tetanus, pregnant women may be immunized in order to transfer antibodies to the child.



High-risk groups that are more likely to develop complications from infection, may receive antibody preparations to prevent these infections or to reduce the severity of symptoms.



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