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. 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. It stems from the individual immunity that may be gained through natural infection or through vaccination. The term herd immunity was initially introduced more than a century ago. 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.1
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CME Disclosure Statement: Unless noted, all individuals in control of content reported no relevant financial relationships. If applicable, all relevant financial relationships have been mitigated.
Corresponding Author: Saad B. Omer, MBBS, MPH, PhD, Yale University, 1 Church St, New Haven, CT 06510 (firstname.lastname@example.org).
Published Online: October 19, 2020. doi:10.1001/jama.2020.20892
Conflict of Interest Disclosures: Dr Yildirim reported being a member of the mRNA-1273 Study Group. No other disclosures were reported.
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