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Herd immunity represents the concept of protection of the population from infection which is achieved by the presence of immune individuals.82 Herd immunity is the resistance to the spread of a contagious disease within a population that results if a sufficiently high proportion of individuals are immune to the disease, either through vaccination or contracting the disease, called natural infection.82 Once herd immunity is achieved, the spread of disease from person to person becomes unlikely. As a result, the whole community becomes protected, not just those who are immune. For a disease to spread, a sufficiently high percentage of the population must be capable of getting a disease to ensure that infectious individuals are likely to come into contact with susceptible individuals. This is called a threshold proportion. If the proportion of the population that is immune to the disease is greater than this threshold, the spread of the disease will decline. In the aforementioned case, the herd immunity threshold would be reached.83
The percentage of a community that needs to be immune to a disease to achieve herd immunity varies from disease to disease. The more contagious a disease is, the greater the proportion of the population that needs to be immune to the disease to stop its spread. For measles, the threshold is 94%, for COVID‑19, while it is unknown, it is believed to be slightly lower. Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases (NIAID) since 1984 estimated the overall herd immunity threshold, “I would say 50% would have to get vaccinated before you start to see an impact,” Fauci said. “But I would say 75 to 85% would have to get vaccinated if you want to have that blanket of herd immunity."84
Herd immunity is impacted by the basic reproduction number (R0). The R0, pronounced “R-naught,” is also called the basic reproduction ratio/rate or the basic reproductive rate. R0 is an epidemiologic metric used to describe the contagiousness or transmissibility of infectious agents.85 In a susceptible, unvaccinated population, each measles carrier will infect an estimated 12 to 18 other people. In the language of epidemiology, the R0 of measles is 12 to 18."Super Script - 86" While there is still some discussion about the R0 of the COVID‑19 coronavirus, without interventions is generally estimated to be between 2.2 and 2.7. If the R0 falls below 1, that means the outbreak is subsiding as fewer people are infected. If it remains above 1, the outbreak is ongoing with increasing numbers of individuals becoming infected. The goal of social distancing practices and vaccinations are to curb the epidemic by driving the R0 of the coronavirus below 1.
Image Source: Wikimedia Commons. File:Herd immunity.svg
NIAID: The top box shows an outbreak in a community in which a few people are infected (shown in red) and the rest are healthy but unimmunized (shown in blue); the illness spreads freely through the population. The middle box shows a population where a small number have been immunized (shown in yellow); those not immunized become infected while those immunized do not. In the bottom box, a large proportion of the population have been immunized; this prevents the illness from spreading significantly, including to unimmunized people. In the first two examples, most healthy unimmunized people become infected, whereas in the bottom example only one fourth of the healthy unimmunized people become infected.