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Herd Immunity and Implications for SARS-CoV-2 Control
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In this article, the authors discuss herd immunity, including its definition and examples of successful vaccine-induced herd immunity.
- Implications in the setting of COVID-19 infection are also discussed.
abstract
This abstract is available on the publisher's site.
Access this abstract nowHerd 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.
Eradication 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.
Additional Info
Disclosure statements are available on the authors' profiles:
Herd Immunity and Implications for SARS-CoV-2 Control
JAMA 2020 Oct 19;[EPub Ahead of Print], SB Omer, I Yildirim, HP FormanFrom MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
Hope for the herd
This week, the first estimates of COVID-19 vaccine efficacy—emerging from an interim analysis in a phase III trial—were released.1 The >90% efficacy for this mRNA vaccine was significantly higher than my expectations and provides hope. In addition, this brings the concept of herd immunity back onto center stage.
Herd immunity refers to the “protection of susceptible individuals against an infection when a sufficiently large proportion of immune individuals exists in a population.”2 Basically, this means that the likelihood of an infected individual coming into contact with a susceptible individual is reduced to below the level needed to sustain a chain of transmission. An outbreak slows; a pandemic ends.
A succinct review of herd immunity is available and helps in the basic understanding of the interplay between infectivity and the requisite herd immunity threshold.2 For highly transmissible viruses, like measles, very high immunity is needed to prevent outbreaks. For SARS-CoV-2, with a basic reproduction number (Ro) between 2 and 3 (ie, 2 to 3 new cases generated per existing cases in a population of susceptible individuals), the estimated herd immunity threshold is around 60%.
We can attain the threshold through infection; the consequences in terms of morbidity and mortality, however, are untenable. We can also arrive at this magical place through vaccination, and this is where news of preliminary estimates of high efficacy are crucial. We will need high vaccine efficacy and high COVID-19 vaccine coverage rates to carry us over the heard immunity threshold. The vaccine efficacy part is on the scientists and manufacturers creating the vaccines. Vaccine coverage is on us.
Primary care clinicians and other healthcare workers will be among the first to receive vaccines for COVID-19. By our example, we hope to lead our patients, families, and friends to acceptance. Better understanding of concepts like herd immunity is important across the whole of our population. Accordingly, patient information, such as the JAMA Patient Page on herd immunity, can be a worthwhile tool in your upcoming vaccine discussions.3
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