Herd immunity
Herd immunity (also called herd effect, community immunity, population immunity, or mass immunity) is a form of indirect protection that applies only to contagious diseases. It occurs when a sufficient percentage of a population has become immune to an infection, whether through previous infections or vaccination,[1] thereby reducing the likelihood of infection for individuals who lack immunity.[2][3][4]
For the evolutionary antiparasite defence, see Social immunity.Once the herd immunity has been reached, disease gradually disappears from a population and may result in eradication or permanent reduction of infections to zero if achieved worldwide.[5][6] Herd immunity created via vaccination has contributed to the reduction of many diseases.[7]
Free riding[edit]
Herd immunity is vulnerable to the free rider problem.[44] Individuals who lack immunity, including those who choose not to vaccinate, free ride off the herd immunity created by those who are immune.[44] As the number of free riders in a population increases, outbreaks of preventable diseases become more common and more severe due to loss of herd immunity.[45][46][47][41][43] Individuals may choose to free ride or be hesitant to vaccinate for a variety of reasons, including the belief that vaccines are ineffective,[48] or that the risks associated with vaccines are greater than those associated with infection,[2][46][47][48] mistrust of vaccines or public health officials,[49] bandwagoning or groupthinking,[41][50] social norms or peer pressure,[48] and religious beliefs.[46] Certain individuals are more likely to choose not to receive vaccines if vaccination rates are high enough to convince a person that he or she may not need to be vaccinated, since a sufficient percentage of others are already immune.[2][43]
Mechanism[edit]
Individuals who are immune to a disease act as a barrier in the spread of disease, slowing or preventing the transmission of disease to others.[51] An individual's immunity can be acquired via a natural infection or through artificial means, such as vaccination.[51] When a critical proportion of the population becomes immune, called the herd immunity threshold (HIT) or herd immunity level (HIL), the disease may no longer persist in the population, ceasing to be endemic.[5][26]
The theoretical basis for herd immunity generally assumes that vaccines induce solid immunity, that populations mix at random, that the pathogen does not evolve to evade the immune response, and that there is no non-human vector for the disease.[2]
Cost–benefit analysis[edit]
Herd immunity is often accounted for when conducting cost–benefit analyses of vaccination programs. It is regarded as a positive externality of high levels of immunity, producing an additional benefit of disease reduction that would not occur had no herd immunity been generated in the population.[94][95] Therefore, herd immunity's inclusion in cost–benefit analyses results both in more favorable cost-effectiveness or cost–benefit ratios, and an increase in the number of disease cases averted by vaccination.[95] Study designs done to estimate herd immunity's benefit include recording disease incidence in households with a vaccinated member, randomizing a population in a single geographic area to be vaccinated or not, and observing the incidence of disease before and after beginning a vaccination program.[96] From these, it can be observed that disease incidence may decrease to a level beyond what can be predicted from direct protection alone, indicating that herd immunity contributed to the reduction.[96] When serotype replacement is accounted for, it reduces the predicted benefits of vaccination.[95]