Gahyun Lee, Hong Kong International School, Hong Kong, SAR China
Summary
Amid the COVID-19 pandemic, herd immunity is often mentioned (for instance, by politicians) when discussing the end of the pandemic. However, the history of herd immunity and its previous applications to other infectious diseases is essential to understanding herd immunity. Utilizing herd immunity as an indicator of the end of the pandemic is challenging. Due to the many assumptions involved in determining herd immunity, this value holds many limitations in its application to the real world. Therefore, rather than only relying on reaching herd immunity, it is essential to take continuous action through measures such as vaccination programs, mask-wearing, and physical distancing to mitigate the severity of the COVID-19 pandemic.
In December of 2021, Dr. Anthony Fauci, the White House Chief Medical Advisor, said that it was “entirely conceivable, and likely” that “the country could reach a level of population immunity in which the virus [SARS-CoV-2] is no longer dominating people’s lives” (CBS News, 2021). However, what does “population immunity” (also known as herd immunity) mean, and is it an attainable indicator as to the end of the COVID-19 pandemic?
Herd immunity is the effect of indirect protection from a virus created when a relatively large proportion of the population is partially immune to the pathogen. It begins when many individuals who have developed immunity to a virus protect a larger population to the point where cases of the specific virus infection decline. Veterinarian George Potter first discovered this concept in 1918 (McDermott, 2021). The bacterium Brucella abortus caused spontaneous abortions in cattle in Kansas. Potter advised farmers against killing cows that caught the disease so that once enough cows recovered from the disease, population immunity would occur. The underlying concept is that there are enough individuals with immunity (through natural infection or vaccination); herd immunity creates a protective wall against those susceptible to infection.
The significance of herd immunity to humans was uncovered with the smallpox outbreak. Smallpox was a highly infectious disease caused by the variola virus that induced skin rashes and fevers. It was highly transmissible by human contact (R naught value of 3) (McDermott, 2021). In 1980, the World Health Organization declared the eradication of smallpox (World Health Organization, n.d.). The eradication of smallpox can be attributed primarily to mass vaccination programs, resulting in herd immunity where non-immune individuals were shielded. Another example of herd immunity is the near eradication of measles (an acute respiratory illness). In the United States, 90% of children are vaccinated against measles. Therefore, even with reported localized cases, measles has not caused a worldwide pandemic because the general population is vaccinated against measles or protected by herd immunity.
The basic reproduction number (R0 or R naught) is an important concept when discussing the virulence of a disease. The R naught value represents the average number of secondary infections caused by an infected individual, given that the population has no immunity. For instance, if the R naught value is 3, one individual will pass on the virus to three susceptible people in a stable environment. As modeled in Figure 1, a population with herd immunity results in lower successful transmissions and greater numbers of indirectly protected individuals. The R naught value can help determine the herd immunity threshold where the proportion of vulnerable individuals is below the threshold for transmission. Theoretically, reaching the threshold value indicates herd immunity has been achieved. Therefore, the higher the disease’s transmissibility, the higher the R naught value and immunity threshold. Yet we must consider that the R naught and threshold values continue to vary as infection rates evolve through a pandemic (Fisher, 2020).
Figure 1. The difference in response to an infected individual entering population that is naive or has herd immunity (Randolph, H. E., & Barreiro, L. B, 2020).
While the concept of herd immunity has utility, it is essential to recognize its limitations. Herd immunity is based on many assumptions since it is a theoretical concept dependent on mathematical models. The first fundamental assumption for herd immunity is that populations arehomogenous with random contacts. where everyone has an equal probability to contact a person infected with the virus. However, the reality is that people mix non-randomly by traveling routine paths, visiting familiar locations, and contacting people within communities and demographic groups. Moreover, superspreading events like large cultural or social gatherings create an environment of high transmission rates, increasing the R naught value temporarily (Majra, 2021). Another assumption is that individuals are equally affected in contracting and spreading the virus. Rather, individuals ill with chronic diseases are at higher risk of contracting the virus and suffering severe symptoms due to comorbidity. Finally, being vaccinated or naturally recovered from COVID-19 does not guarantee complete immunity, as there is a small, but possible chance of reinfection. In fact, the Omicron variant has increased chances of reinfection by 10% from November 2021 to Feburary 2022 (Mallapaty, 2022).
Furthermore, nonrandom vaccine distributions due to vaccine hesitancy present limitations when establishing herd immunity (Fine et al., 2011). Often, distrust in governments and vaccines results in vaccine hesitancy. According to research by Carnegie Mellon University and the University of Pittsburgh on COVID-19 vaccine hesitancy, groups differing by political beliefs, race, age, and education level indicated varying levels of vaccine hesitancy (Heuring, 2021). This study discovered that generally, in North America, those residing in areas with greater support towards Donald Trump in the 2020 election, younger African Americans, older Caucasians, and groups with less education (below high school diploma) displayed greater hesitancy. Therefore, addressing misinformation and providing accessible healthcare is essential.
According to the Centers for Disease Control and Prevention (CDC), being unvaccinated increases the chances of being infected with COVID-19 by ten times compared to fully vaccinated individuals with booster shots, resulting in onward transmission (Centers for Disease Control and Prevention, n.d). Although the immunity threshold is a nuanced estimate, all COVID-19 vaccines and their effectiveness cannot be denied. Herd immunity is a difficult value to determine. Therefore, the primary purpose of establishing herd immunity through vaccinations should be to protect immunocompromised, including the elderly or young, since they cannot be vaccinated. We must cooperate in a collective effort to get vaccinated and continue behaviors such as hand washing, social distancing, and mask-wearing to fight this ongoing pandemic, as variants evolve such as Delta and Omicron. Ultimately, policymakers should focus on the importance of protecting vulnerable groups in communities and aim to close disparities in education and health care access to fight the pandemic.
References
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Centers for Disease Control and Prevention. (n.d.). CDC Covid Data tracker. Centers for Disease Control and Prevention. Retrieved February 7, 2022, from https://covid.cdc.gov/covid-data-tracker/#rates-by-vaccine-status
Fine, P., Eames, K., & Heymann, D. L. (2011). "Herd immunity": A rough guide. Clinical Infectious Diseases, 52(7), 911-916. https://doi.org/10.1093/cid/cir007
Fisher, M. (2020, April 23). R0, the Messy Metric That May Soon Shape Our Lives, Explained. New York Times. Retrieved October 20, 2021, from https://www.nytimes.com/2020/04/23/world/europe/coronavirus-R0-explainer.html
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Majra, D., Benson, J., Pitts, J., & Stebbing, J. (2021). SARS-CoV-2 (COVID-19) superspreader events. Journal of Infection, 82(1), 36-40. https://doi.org/10.1016/j.jinf.2020.11.021
Mallapaty, S. (2022, February 16). COVID reinfections surge during Omicron onslaught. Nature. Retrieved April 26, 2022, from https://www.nature.com/articles/ d41586-022-00438-3#:~:text=Most%20reinfections%20occurred%20about%20one,high%2C%2 0at%20around%2088%25.
McDermott, A. (2021). Core concept: Herd immunity is an important—and often misunderstood—public health phenomenon. Proceedings of the National Academy of Sciences, 118(21), e2107692118. https://doi.org/10.1073/pnas.2107692118
Randolph, H. E., & Barreiro, L. B. (2020). Herd Immunity: Understanding COVID-19. Immunity, 52(5), 737-741. https://doi.org/10.1016/ j.immuni.2020.04.012
World Health Organization. (n.d.). Smallpox. World Health Organization. Retrieved February 7, 2022, from https://www.who.int/westernpacific/health-topics/smallpox
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