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JAMA Insights
October 19, 2020

Herd Immunity and Implications for SARS-CoV-2 Control

Author Affiliations
  • 1Yale Institute for Global Health, New Haven, Connecticut
  • 2Departments of Internal Medicine and Epidemiology of Microbial Diseases, Yale Schools of Medicine and Public Health, New Haven, Connecticut
  • 3Section of Infectious Diseases and Global Health, Department of Pediatrics, Yale School of Medicine, New Haven, Connecticut
  • 4Yale School of Public Health, New Haven, Connecticut
  • 5Yale School of Management, New Haven, Connecticut
JAMA. 2020;324(20):2095-2096. doi:10.1001/jama.2020.20892

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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    4 Comments for this article
    Estimated Infections in US
    Tom Kocsis, BS ME, MS Telecom | Private Citizen
    The statement "assuming that less than 10% of the population has been infected so far" is open to question. That statement is made based on a study of "Prevalence of SARS-CoV-2 antibodies in a large nationwide sample of patients on dialysis in the USA" (the paper's reference 10).  That seems a very limited and narrow group of individuals from which to base the 10% assumption. Further, the existence of antibodies is not the sole confirmation of whether an individual had a SARS-CoV-2 infection previously. I continue to meet people who believe they had SARS-CoV-2 in the Jan-Apr 2020 timeframe before testing was widely available. Some of those folks went for an antibody test later in the Aug-Sep timeframe and their tests came back negative. I think there's still much to be learned on what is a good test for prior infection. Further, there's the entire subject of "asymptomatic" individuals.  How many of these individuals never even go in for a test later on since they won't know or suspect they've been infected? I do appreciate this article and its insights despite the criticism stated above and hope that the research community discovers more about this virus and how it is resisted by the human immune system in asymptomatic individuals and that it would lead to a much more effective and revealing test to possibly arrive at a more accurate picture of infection across the US. Also, I'd really like to see new research on how this virus is mutating over time and whether its "deadliness" is declining based on the death rates that we see in the US as having declined and flattened since early Aug while the number of infections has continued to increase at a high rate. Is this due to the virus becoming less "deadly" as it spreads thru the population or is it because of the treatments that are being administered now? Appreciate all the research and work to battle this new virus.
    The Myth of SARS-CoV-2 Control
    Paul von Ebers, MBA Health Administration | Health care consultant, insurance executive
    Omir, Yildrim, and Forman argue that herd immunity in the absence of an effective vaccine is not a realistic strategy for SARS-CoV-2 control. They leave open the question of what a realistic strategy would be. Common wisdom is that masking, handwashing, social distancing, and avoiding large gatherings is the strategy that will bring an end to the pandemic. This argument has merit, given reductions in infection rates following the implementation of these strategies in various places around the world. However, a key, unanswered question is whether this strategy is sustainable.

    The authors,
    for example, state what appears to be true: "Importantly, even small deviations from protective levels can allow for significant outbreaks due to local clusters of susceptible individuals." Human behavior has probably changed more dramatically in response to Covid than any other health issue in recent history and yet it has not been enough to stop the virus. Renewed outbreaks are happening even in locations where significant restrictions on person to person contact were put in place for considerable periods of time. Indeed, now the White House coronavirus task force is warning about small family gatherings for holidays as a major risk factor.

    Assuming we are as far from herd immunity as the authors state, it appears that the authors are arguing for protracted periods of social isolation and reduced normal human behavior unless an effective vaccine is developed. And the authors have some doubt about how soon and how effective a vaccine will be.

    The authors argue that protecting the vulnerable while allowing the virus to burn out in the less vulnerable population is not possible. Even if they are right, they fail to consider social determinants of health impacted by the common wisdom strategy. We know that unnecessary non-Covid deaths are occurring due to reductions in needed care. Preventive care and chronic disease management has been reduced. Low income children are losing valuable school experience and food support due to school closings. Low income parents are losing their incomes due to business closures. Income and health disparities are growing as a direct result of the social isolation strategy. Elderly patients at the end of life without Covid are dying without the comfort of their families. Covid restrictions are not only "an inconvenience" as many public health experts have claimed. They are affecting the morbidity, mortality and lifetime economic prospects (which is correlated to health) of millions of people worldwide.
    Is herd immunity even possible
    Garreth Debiegun, MD | Maine Medical Center
    This article states that infection-based herd immunity has never actually been achieved. I was aware of this before and hypothesize that it can never be achieved because, as the proportion of susceptible individuals decreases, so will the prevalence of the virus. This will continue until a balance is left that enables the virus to persist in the community at lower levels. Otherwise many viruses and other illnesses we see regularly would cease to exist. I’m not an epidemiologist or virologist, is this true? Is infection-achieved herd immunity actually possible?
    SARS-CoV-2: Natural Herd Immunity and Complications
    Gary Ordog, MD, DABMT, DABEM | County of Los Angeles, Department of Health Services, (retired)
    Thank you for the excellent article and comments, being both timely and important. Natural herd immunity probably will not stop the virus. The answer is a good vaccine, several of which that are high efficiency should be available soon. There must be a major effort to implement worldwide vaccination administration next. These discussions, so far, have not mentioned a major reason for advising against natural herd immunity. The COVID-19 mortality rate is quite obvious, but the other concerning metrics of complications were not presented. A major reason to avoid this virus is the high percentage (estimated at least 97%) of complications including immunological, neurological, respiratory, renal, and most other systems, with the potential for long term sequelae. Prudent advice at this time seems to be to avoid exposure this (and probably any) virus, avoid natural herd immunity, and rely upon the administration of good vaccines.
    Thank you and stay safe.