For example, even with modest infection fatality ratios, a new pathogen will result in substantial mortality because most, if not all, of the population would not have immunity to the pathogen. However, such a strategy is fraught with risks. 2 Assuming no population immunity and that all individuals are equally susceptible and equally infectious, the herd immunity threshold for SARS-CoV-2 would be expected to range between 50% and 67% in the absence of any interventions.Īn infection-based herd immunity approach (ie, letting the low-risk groups become infected while “sequestering” the susceptible groups) has been proposed to slow the spread of SARS-CoV-2. Since the beginning of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, most of the studies estimated the SARS-CoV-2 R 0 to be in the range of 2 to 3. A highly communicable pathogen, such as measles, will have a high R 0 (12-18) and a high proportion of the population must be immune to decrease sustained transmission. 2, 3 The effective reproduction number incorporates partially immune populations and accounts for dynamic changes in the proportion of susceptible individuals in a population, such as seen during an outbreak or following mass immunizations. In the simplest model, the herd immunity threshold depends on the basic reproduction number (R 0 the average number of persons infected by an infected person in a fully susceptible population) and is calculated as 1 − 1/R 0 ( Figure). If the proportion of immune individuals in a population is above this threshold, current outbreaks will extinguish and endemic transmission of the pathogen will be interrupted. The herd immunity threshold is defined as the proportion of individuals in a population who, having acquired immunity, can no longer participate in the chain of transmission. 1Įradication of smallpox and sustained reductions in disease incidence in adults and those who are not vaccinated following routine childhood immunization with conjugated Haemophilus influenzae type B and pneumococcal vaccines are successful examples of the effects of vaccine-induced herd immunity. In the latter half of the 20th century, the use of the term became more prevalent with the expansion of immunization programs and the need for describing targets for immunization coverage, discussions on disease eradication, and cost-effectiveness analyses of vaccination programs. The term herd immunity was initially introduced more than a century ago. It stems from the individual immunity that may be gained through natural infection or through vaccination. In other words, herd immunity is the inability of infected individuals to propagate an epidemic outbreak due to lack of contact with sufficient numbers of susceptible individuals. Herd immunity, also known as indirect protection, community immunity, or community protection, refers to the protection of susceptible individuals against an infection when a sufficiently large proportion of immune individuals exist in a population.
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