The University of Tennessee, Knoxville.
Herd Immunity in Tennessee

A Policy Brief by the Howard H. Baker Jr. Center for Public Policy in Partnership with the Coronavirus-19 Outbreak Response Experts (CORE-19)

July 22, 2020
Tennessee State Capitol and Flag
Using publicly available data from emerging research on COVID-19, this brief was written and reviewed by the Coronavirus-19 Outbreak Response Experts (CORE-19) at the University of Tennessee, Knoxville. It contains an explanation of herd immunity and its policy implications.

Herd Immunity in Tennessee

When combatting a pandemic, such as COVID-19, suppression and mitigation strategies are often the only methods available to prevent disease spread and limit excess mortality in the near-term. However, intensive non-pharmaceutical interventions (i.e. ‘Stay at Home’ orders, school closures, etc.) are widely considered an unsustainable solution in the long-term, since the socioeconomic costs associated with their continued use will at some point begin to supersede the benefits. Thus, the point at which certain public health interventions should be relaxed or removed crucially depends on three factors:
  • Testing capacity and contact tracing efficiency
  • Development or availability of therapeutics or vaccinations
  • Progress towards herd immunity
This policy brief provides an overview of the scientific literature concerning herd immunity. We examine the prospect of herd immunity in Tennessee and consider the costs and benefits associated with reaching a herd immunity threshold.

Defining Herd Immunity 

In a fully susceptible population where no individuals have immunity, an infectious pathogen can proliferate among a population at an exponential rate. Herd immunity (also known as protective or community immunity) refers to a situation in which a large proportion of a population acquires biologic immunity to an infectious disease, thereby slowing disease transmission and providing indirect protection to individuals who remain fully susceptible to infection.
Figure 1: A Simple Illustration of How to Establish Herd Immunity   
Figure 1: A simple illustration of how to establish herd immunity
Graphic produced by the Vanderbilt School of Medicine’s Health Policy & Public Health COVID-19 Advisory Panel. 
Such protective immunity, where indirect protection is conferred to those who remain fully susceptible to infection should they ever be exposed, is particularly advantageous for high-risk groups, such as the very young or old and immunocompromised. The herd effect also underpins many global public health campaigns aimed at decreasing transmission of infectious disease and disease eradication. Common examples include polio, measles, mumps, and a variety of other illnesses that presented a far greater threat to prior generations.
There are three predominant modes through which a population can establish herd immunity: widespread infection, widespread vaccination, or a combination of the two. However, in a completely naive population, where all individuals are initially susceptible to an emerging infectious disease like COVID-19, herd immunity is rarely achieved in the near-term or in the absence of an effective vaccination program. 

Calculating the Herd Immunity Threshold

While the threshold can vary by disease, scholars generally indicate between 60% - 80% of a population must have immunity in order to provide sufficient indirect protection to the remaining susceptible population. 
The proportion of a population that must have immunity in order to reach herd immunity varies by disease according to its basic reproduction number (R0), which denotes the number of new infections resulting from a single infected individual in a fully susceptible population. The precise herd immunity threshold then is calculated as: 1 - 1 / R0. Thus, as the basic reproduction number (R0) increases, indicating a more infectious disease, more individuals must be immune in order to slow transmission. Figure (2) shows this relationship.
Figure 2: Relationship between R0 and the Herd Immunity Threshold
Figure 2: Relationship between R0 and the Herd Immunity Threshold
Graphic produced by authors.
However, the basic reproduction number (R0) is a theoretical value and notoriously hard to estimate reliably, especially in early phases of spread, so immunity threshold calculations using this value alone can be problematic. Other calculations utilize the effective reproductive number (Rt), which is less theoretical, more reflective of transmission rates, and based on current data. 
Multiple studies place the basic reproduction number (R0) value for the COVID-19 virus between 2 and 6, translating to a herd immunity threshold between roughly 50% and 80% population immunity. It is important to note, however, calculations can vary greatly across studies. Scholars often avoid precise estimations of the immunity threshold, in part, because methodology, data and a range of epidemiological factors can directly or indirectly influence R0 and, therefore, the required threshold.

Epidemiological Considerations For Establishing
Herd Immunity

The concept of herd immunity critically relies on the assumption that individuals who recover from COVID-19 develop IgG and IgM antibodies that confer some level of protection from reinfection. To date, most of the scientific literature regarding whether such antibodies confer immunity is based on past coronaviruses and, according to the CDC, it is still unclear whether antibodies confer immunity. But leading experts suspect the presence of antibodies likely confers an individual at least some level of protection from reinfection in the short-term, though researchers stress that this may not always hold true.  
The duration of protection provided by COVID-19 antibodies is still scientifically unclear.
Population Heterogeneity & Non-random Mixing
Large epidemics can often be better understood as a conglomerate of smaller, regional epidemics among different subgroups. This was the central idea from an important paper by Fox et al. in 1972 and why many public health scholars argue population heterogeneity, or how a population mixes or interacts within and between social groups, plays a critical role in predicting the requisite herd immunity threshold. The exact role social networks and population heterogeneity play, however, can be difficult to discern. Estimations which aim to account for it often confound the value of R0, resulting in true thresholds much larger or smaller than originally predicted.
For example, one recent study seeking to consider the impact of social networks and non-random social interaction based on an R0 value of 2.5 (which normally equals a 60% threshold) places the threshold at 43%. Another study, using data from the SARS-CoV-1 outbreak in 2002, finds that the herd immunity threshold for COVID-19 (SARS-CoV-2) could be as low as 10-20% once accounting for individual-level variation in susceptibility and exposure to infection, though this is an unusually low estimate.
Free Riding
Free riding can also influence the effectiveness of vaccine programs and, indirectly, the process of establishing herd immunity. When an individual perceives a vaccine as high-risk, they may rely on the community to vaccinate while not receiving the vaccine themselves. If this mindset becomes commonplace, even to a relatively small extent, the community can fall short of the required immunity threshold and thus remain susceptible to large outbreaks.
Non-Random Vaccination
Non-random vaccination programs are designed to target groups that transmit the disease at the highest rate, mitigating some but not all of the risk posed to the most vulnerable population groups. For example, vaccination programs specifically targeting school-aged children could provide population-level protection, especially if school-aged children are known to be the predominant transmitters of a disease. In practice, however, this does not always hold true. 
Vaccine Effectiveness & Imperfect Immunity 
While understanding who and how many people must be vaccinated to reach herd immunity is necessary, it is also vital to understand the effectiveness of vaccination programs. Vaccines are weakened or synthetic versions of pathogens that elicit an immune response causing the production of antibodies. Possessing antibodies allows an individual to mount a response against the pathogen immediately upon exposure. Early research on herd immunity assumes that any vaccination provides total immunity to the individual, but more recent research suggests that’s not always the case. Imperfect Immunity indicates that a vaccine may not provide immunity and, in some cases, requires regular booster shots. For example, the seasonal flu vaccine must be administered on a yearly basis because the virus causing the seasonal flu is particularly prone to mutation. Additionally, the vaccine which protects against diphtheria, tetanus, and pertussis must be administered in a series of 4 shots to children in order for them to build immunity. Booster vaccines are also required every 10 years for teens and adults as immunity can wear off over time. 
Antibodies & Immunity
Understanding the antibodies to COVID-19 is key to understanding if and when a vaccine will be an effective course of action to reach herd immunity. It is presumed that once an individual develops antibodies, they will always be immune to that disease. This was a common assumption with chickenpox, but the virus can resurface later in life as shingles. Research on antibodies to COVID-19 is still in the early stages, and thus we can not assume that the presence of antibodies will confer lifelong immunity. Antibodies to COVID-19 seem to appear in patients in the middle to late stage of infection. IgM antibodies serve as the primary immune response while IgG antibodies are known to linger in the bloodstream, possibly providing long term immunity. Figure (3) demonstrates when each antibody can be detected and its estimated lifespan.
Figure 3: The Lifecycle of Antibody Testing 
Figure 3: The LIfecycle of Antibody Testing
Graphic produced by CliniSciences
Experimental treatment options involving collecting plasma containing antibodies to COVID-19 from previously ill patients and transfusing the plasma to patients currently suffering from the virus have proven to be effective in some trials. Figure (4) illustrates this process.
Figure 4: Schematic of the use of convalescent sera for COVID-19
Figure 4: schematic of the use of convalescent sera for COVID-19
Antibody testing, also known as serology testing, can also help researchers understand how many individuals in a community have previously been infected with the virus. However, according to the Director of the National Institute of Allergy and Infectious Diseases Dr. Anthony Fauci, there are “examples of people who clearly were infected who are antibody negative,” while others have very robust antibody responses, meaning the concentration of antibodies can vary significantly by person. 
Additionally, research is still unclear on how long antibodies exist in those who have been infected. Many experts, including Fauci, also question the durability of immunity, as the history of common coronaviruses suggests the range of protection typically only lasts “from 3 to 6 months to almost always less than a year.” There have been reports of some patients in Japan becoming reinfected with COVID-19 weeks after initial recovery. Recent studies show that antibody levels are high weeks after infection before they begin to decline, although if the patient suffered a more severe case, their antibodies may last longer. In some cases, certain antibody responses are correlated with a worse disease outcome. It is also still unclear regarding the level of antibodies that is required to fight off infection. The “lifetime” of antibodies to COVID-19 will directly impact the effectiveness of a vaccine to COVID-19. Given the many unknowns, it is vital that treatment and prevention strategies for COVID-19 include a variety of options, including plasma transfusions and vaccinations. 

Prospects for Herd Immunity in Tennessee

As the United States commenced the process of reopening the economy and with many states lifting stay-at-home orders in favor of more mitigation-oriented policies, such as mask mandates, the concept of herd immunity has become increasingly prevalent in the public discourse surrounding COVID-19. However, barring any drastic change in the situation or rapid development and deployment of an effective vaccine for COVID-19, the prospect of reaching herd immunity anytime in the near future is unlikely. 
Figure (5) shows population immunity levels in Tennessee (as of July 20) based on the currently reported case counts, assuming recovery from the virus confers effective immunity to the infected individual. According to CDC officials, antibody (serology) testing suggests the true number of infections in the United States may be up to 10 times greater than what is reflected by official cases counts. As shown in Figure (5), even if the true number of infections is 10 times greater than the official case count, the population immunity level in Tennessee would still not exceed 20%.
Figure 5: Population Immunity Level Estimations for Tennessee
figure 5 population immunity level estimations for Tennessee
Graphic produced by the author.
In addition, reaching herd immunity in the short-term, when no vaccination is available, would likely come with tremendous societal and economic costs. Some basic math can help illustrate this dynamic. So, take the following assumptions to help consider the consequences of reaching herd immunity in the absence of a vaccine in Tennessee:
  • Herd immunity threshold of 60%
  • Population size of 6,886,369 (2020 Boyd Center estimate)
  • Value of a Statistical Life equal to $9.4 million (EPA estimate updated using 2019 CPI)
  • Hospitalization rate of 5% (current TN hospitalization rate)
  • Hospitalization cost per person between $31,105 - $75,902 (CEA estimate for influenza using the population-weighted average for medical costs and lost productivity)
  • Infection fatality rate varies between 0.2% and 1%
Infection fatality rate (IFR) refers to the number of deaths caused by a disease as a proportion of total infections (symptomatic and asymptomatic), which is by definition lower than the case fatality rate (CFR), or the number of deaths caused by a disease as a proportion of total documented cases. A recent meta-analysis of 24 studies with published research data estimates an IFR between 0.50-0.78% for COVID-19.  However, the IFR can and often does vary significantly across time, region, and locality.  
Figure 6: Projected Economic Cost of Establishing Herd Immunity Compared to TN Real GDP 
figur e6: projecrted economic cost of establishing immunity compared to TN Real GDP
Scenario #1: Infection fatality rate (IFR) = 0.2% →  
4,131,821 infections
206,591 hospitalizations
8,264 fatalities
Total Economic Cost: $84.2 - $93.4 billion (27% - 28% of TN GDP)
Scenario #2: Infection fatality rate (IFR) = 0.6% → 
4,131,821 infections
206,591 hospitalizations
24,791 fatalities
Total Economic Cost: $239.5 - $248.7 billion (72% - 75% of TN GDP
Scenario #3: Infection fatality rate (IFR) = 1% → 
4,131,821 infections
206,591 hospitalizations
41,318 fatalities
Total Economic Cost: $394.8 - $404.1 billion (119% - 122% of TN GDP) 

It is important to note that the above scenarios are overly simplified calculations. However, given the clinical and economic uncertainties surrounding COVID-19, such calculations help illustrate the magnitude of the potential costs associated with establishing herd immunity in the absence of a vaccine.
Any scenario in which Tennessee establishes herd immunity prior to the introduction and widespread deployment of a vaccine is unlikely and would come with a tremendous human and economic cost.

Policy Implications & Recommendations

  • Since many asymptomatic or moderate cases of COVID-19 likely went undetected during the early days of the pandemic, widespread and sustained serological testing (commonly known as antibody testing) can help determine true infection rates and thereby provide better insight into a given population’s progress towards herd immunity
  • To provide insight into the nature of COVID-19 immunity, policymakers and public health officials should closely monitor the frequency of reinfection among individuals who previously contracted SARS-CoV-2.
  • Considerable progress has been made towards developing a vaccine for SARS-CoV-2 in recent weeks. However, should an effective vaccine become available within the next 12 to 18 months, public opinion polling suggests a significant percentage of Americans wouldn’t get the vaccine. All levels of government, but particularly state and local governments, should prioritize the development of vaccine deployment strategies aimed at boosting “vaccine confidence” and combating online misinformation. In addition to digital and social media campaigns, public health officials should consider real-world nudges, such as phone call reminders and engaging community groups.  
  • Robust surveillance programs monitoring vaccination coverage are critical to understanding population immunity levels. This will likely require robust public-private partnerships, as evidenced by the current reporting structure for identified cases of COVID-19 in Tennessee. State and local governments should work to establish these partnerships preemptively to ensure accurate data is available following the deployment of a vaccine.
Send any additional questions to the CORE-19 team. | 865-321-1299

Coronavirus-19 Outbreak Response Experts (CORE-19) 

Policy Brief Authors
Hancen A. Sale

Hancen A. Sale

Sale is a UTK graduate and research assistant with the Center. He holds a degree in economics with minors in public policy analytics and political science. He has worked on an NSF-funded project regarding rebel group grievances, as well as in supporting The White House's American Workforce Policy Advisory Board. 
Katherine Fulcher

Katherine Fulcher

Fulcher is an undergraduate researcher with the Center. She is a senior majoring in Political Science and Hispanic Studies and minoring in public policy analytics. At the Baker Center, she assists with research support, event management, and media relations.
Coronavirus Outbreak Response Experts (CORE-19)
Steering Committee
Dr. Kathleen Brown

Dr. Kathleen C. Brown, PhD, MPH

Brown is an Associate Professor of Practice in the Department of Public Health and the Program Director for the Master's in Public Health (MPH) degree. Her research focuses on the health and well-being of individuals and communities. She has experience in local public health in epidemiology, risk reduction and health promotion.
Dr. Katie Cahill

Dr. Katie A. Cahill, PhD

Cahill is the Associate Director of the Howard H. Baker Jr. Center for Public Policy. She also is the Director of the Center's Leadership & Governance program and holds a courtesy faculty position in the Department of Political Science. Her area of expertise is public health policy. She leads the Healthy Appalachia project. 
Dr. Kristina Kintziger

Dr. Kristina W. Kintziger, PhD, MPH

Kintziger is an Assistant Professor in the Department of Public Health and the co-Director of the Doctoral Program. She has worked in academia and public health practice. Prior to coming to Tennessee, she served as an epidemiologist and biostatistician at the Florida Department of Health. She is an environmental and infectious disease epidemiologist.
Dr. Matthew Murray

Dr. Matthew N. Murray, PhD

Murray is the Director of the Howard H. Baker Jr. Center for Public Policy. He also is the Associate Director of the Boyd Center for Business and Economic Research and is a professor in the Department of Economics in the Haslam College of Business. He has led the team producing Tennessee's annual economic report to the governor since 1995. 
Dr. Agricola Odoi

Dr. Agricola Odoi, BVM, MSc, PhD

Odoi is a professor of epidemiology at the University of Tennessee College of Veterinary Medicine. He teaches quantitative and geographical epidemiology and his research interests are in population health and impact of place on health and access to health services. He was a public health epidemiologist before joining academia. Odoi is a member of the CORE-19 Steering Committee. 
Dr. Marcy Souza

Dr. Marcy J. Souza, DVM, MPH

Souza is an associate professor and Director of Veterinary Public Health in the UT College of Veterinary Medicine.  Her teaching and research focuses on zoonotic diseases and food safety issues. 
Disclaimer: the information in this policy brief was produced by researchers, not medical or public health professionals, and is based on their best assessment of the existing knowledge and data available on the topic. It does not constitute medical advice and is subject to change as additional information becomes available. The information contained in this brief is for informational purposes only. No material in this brief is intended to be a substitute for professional medical advice, diagnosis or treatment, and the University of Tennessee makes no warranties, expressed or implied, regarding errors or omissions and assumes no legal liability or responsibility whatsoever for loss or damage resulting from the use of information provided.
Howard H. Baker Jr Center for Public Policy
1640 Cumberland Avenue
Knoxville, TN 37996
Phone: 865-974-0931
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