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What Is Herd Immunity?

A public health expert and a medical laboratory scientist explain

Rodney E. Rohde, PhD, MS, SM(ASCP)CM, SVCM, MBCM, FACSc

Rodney E. Rohde, PhD, is University Distinguished Regents’ Professor and chair of the Medical Laboratory Science Program in the College of Health Professions at Texas State University, where he also serves as the associate director of the Translational Health Research Center. He holds certifications as a specialist in virology, specialist in microbiology, and molecular biologist from the American Society for Clinical Pathology. Rohde is considered a subject matter expert in rabies and methicillin-resistant Staphylococcus aureus (MRSA), infection prevention and control, as well as other public health and medical laboratory topics. Rohde has more than 30 years of experience, including a decade with the Texas Department of State Health Services – Zoonosis Control Division and Bureau of Laboratories and the CDC as a visiting scientist from 1992–2002. Find Rohde on LinkedIn, YouTube, and Twitter @RodneyRohde and @TXST_CLS.

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Ryan McNamara

Ryan McNamara earned his Bachelor of Science degree in Biochemistry from New Mexico State University where he studied the evolutionary determinants of dengue virus spillover into the human population. He earned his Doctorate in the Department of Microbiology at the University of Texas - Southwestern Medical Center where his focus was on mechanisms of gene expression and emergence from latency by HIV. He earned his Doctorate with an emphasis on Translational Research. He next became a postdoctoral research associate at the University of North Carolina at Chapel Hill School of Medicine where he focused on AIDS-associated cancers.

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Published:Nov 04, 2021
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The term herd immunity means that enough of a population has gained immunity to stifle a pathogen’s spread. You can think of herd immunity as being similar to fire starting in a field: If the field is dry and filled with weeds, the fire will catch and spread quickly. However, if the field is well-maintained with watering and trimming, the fire will fizzle out. Future embers that might land there will be far less likely to ignite.

The embers are much like SARS-CoV-2, the coronavirus that causes COVID-19.

Herd immunity can theoretically be achieved either through infection and recovery or by vaccination. The danger of trying to achieve herd immunity through infection is that many people will die or be forced to live with post-recovery disabilities. Moreover, research has shown that the immune response resulting from infection does not always provide strong enough long-term protection against COVID-19 and its evolving strains. Thus, public health experts still recommend vaccination against the coronavirus to achieve the strongest and most reliable protection.

When the COVID-19 pandemic erupted, scientists quickly began to develop vaccines so that populations could develop immunity to slow the fire-like spread of the coronavirus. In the meantime, nearly all countries mandated or encouraged social distancing, masking, and other public health measures.

Unfortunately, the disjointed implementation of these efforts, coupled with large-scale surges and the emergence of the highly transmissible Delta variant, has forced public health experts to recalculate what it would take to reach “herd immunity” for COVID-19.

Why herd immunity matters

Prior experience with respiratory pathogens that were comparable to the new coronavirus allowed public health experts to make educated estimates of what would be needed to reach the lower threshold of herd immunity for COVID-19. Initially they believed that around 70 percent of the population would need to be vaccinated to effectively slow or stop the spread of SARS-CoV-2.

But with the Delta variant continuing to spread rapidly around the world, experts revised that estimate. Now, epidemiologists and other public health officials estimate that closer to 90 percent of the US population would need to be vaccinated to reach herd immunity for COVID-19.

Viruses like those that cause polio and measles required decades of education and vaccination programs to achieve herd immunity and to ultimately eliminate them in the US But given that new US cases of COVID-19 continue to number in the tens of thousands daily, it’s become clear that COVID-19 is going to stick around.

There are several reasons it will take some time to achieve COVID-19 herd immunity. The COVID-19 vaccines are currently authorized for some age groups but not others. For perspective, roughly 90 percent of the US population receives the measles, mumps, and rubella vaccine—or MMR—as children, and 93 percent of the population is vaccinated against polio; both of these have been routine childhood immunizations for decades. Since children make up more than 20 percent of US residents, the country likely cannot reach COVID-19 herd immunity without widespread childhood vaccination, even if all eligible adults were vaccinated.

As of November 1, 2021, only 67.8 percent of the total US population ages 12 and up that are vaccine-eligible had been fully vaccinated. Experts have attributed this to multiple factors including vaccine hesitancy and the politicization of the pandemic.

Of course, no vaccine is perfect. Vaccinated people can have breakthrough infections, although the COVID-19 vaccines continue to effectively reduce the most severe cases of COVID-19. In addition, research suggests that those who experience COVID-19 after vaccination may transmit the virus at lower transmission rates than those who are unvaccinated.

Rodney E. Rohde, professor of clinical laboratory science, Texas State University and Ryan McNamara, research associate of microbiology and immunology, University of North Carolina at Chapel Hill

This article is republished from The Conversation under a Creative Commons license. Read the original article.