Herd Immunity
Herd immunity is a public health concept that refers to the protection against infectious diseases that occurs when a significant portion of a population becomes immune, either through vaccination or previous infections. This immunity significantly reduces the likelihood of transmission, thereby protecting even those who are not immune. For herd immunity to be effective, a high percentage of the population must be immunized; if vaccination rates decline, the overall immunity of the community decreases, leading to a higher risk of disease outbreaks.
Diseases like measles, mumps, and polio have seen substantial declines in developed nations due to successful vaccination campaigns that promote herd immunity. However, movements against vaccination pose a serious threat, as seen in outbreaks like the 2015 measles incident in the United States, where unvaccinated individuals became infected due to decreased community immunity. Additionally, the COVID-19 pandemic highlighted the complexities of achieving herd immunity, as it was unclear if recovering individuals maintained sufficient immunity. Overall, maintaining high vaccination rates remains crucial to safeguard public health and prevent the resurgence of previously controlled diseases.
Subject Terms
Herd Immunity
Herd immunity is a form of protection against disease. In herd immunity, the immunization of a large percentage of individuals in a population makes infection of non-immunized individuals extremely unlikely. Should the general rate of immunization fall, more members of the population will become vulnerable and the chance of a disease outbreak will increase.
How Herd Immunity Works
Infectious diseases spread from one infected person to another. This means that for individuals to get a disease, they must come in contact with an infected person. Modern science has allowed humans a variety of ways to avoid getting sick. Vaccinations, exposure to similar viruses, or hereditary biological resistance allow select members of populations to become immune to a disease. However, it is extremely unlikely that each individual in a large population will be immune to every disease. Herd immunity occurs when immunity of a large segment of the population prevents contagious diseases from affecting those in the population who are not immune. Herd immunity prevents diseases from reemerging, even if a small percentage of a population remains vulnerable to the diseases.
When a large percentage of a community is immune to a disease, the percentage of individuals who can bring that disease into the community is reduced. If the percentage of immune individuals is high enough, vulnerable individuals may never interact with other vulnerable individuals, making them effectively immune to the disease. They are protected by herd immunity.
Herd immunity relies on a large percentage of the population being vaccinated or made immune to disease by some other means. On some occasions, individuals will assume that everyone else in the population is vaccinated, and will therefore fail to get vaccinated themselves. This reduces the percentage of immune individuals in the population, making the entire population more vulnerable to disease and decreasing the effectiveness of herd immunity. For this reason, herd immunity should never be a primary means of defense against a disease.
Threats to Herd Immunity
Potentially deadly diseases such as measles, mumps, and polio have been mostly eliminated in developed nations through vaccination and herd immunity. However, anti-vaccination movements threaten this. Fewer people getting vaccinated reduces the overall immunity of the population, making it more likely that a chance introduction of an otherwise controlled disease could turn into a major outbreak. In 2015, the United States experienced a measles outbreak when an infected traveler from outside the country visited a California amusement park. One hundred and eighty-nine people in twenty-four states contracted the disease, which had been eliminated in the United States in 2000. The majority of the cases in the 2015 outbreak were among people who were not vaccinated.
Measles and other contagious diseases can spread quickly when they reach a population where groups of people are not immune through vaccination or herd immunity. Even when infectious diseases have been fully or mostly eliminated from a population, vaccination remains important to protect against reintroduction of the disease.
In the case of the novel coronavirus causing the disease COVID-19 that was first identified in China in late 2019 and began rapidly spreading around the world into 2020, leading to the declaration of a pandemic in March, the idea of accomplishing herd immunity once a large number of people had contracted the virus and recovered was debated. As experts were not immediately sure whether those who had successfully battled COVID-19, thereby developing antibodies, were actually then immune to reinfection, this potential method for herd immunity was not reliable. Additionally, because populations such as that of the United States had grown so large, too many people would need to become infected with and beat the potentially deadly novel coronavirus, overwhelming hospitals, to reach herd immunity. Though the anti-vaccination movement remained a threat, researchers worked diligently on a potential vaccine as a priority, especially for potential herd immunity.
Bibliography
"Herd Immunity." The History of Vaccines, The College of Physicians of Philadelphia, 2018, https://www.historyofvaccines.org/content/herd-immunity-0. Accessed 22 Feb. 2018.
McGinty, Jo Craven. "How Anti-Vaccination Trends Vex Herd Immunity." The Wall Street Journal. Dow Jones, 6 Feb. 2015. Web. 29 Dec. 2015.www.wsj.com/articles/how-anti-vaccination-trends-vex-herd-immunity-1423241871
"Measles (Rubeola)." Centers for Disease Control and Prevention. US Department of Health and Human Services, n.d. Web. 29 Dec. 2015. www.cdc.gov/measles/cases-outbreaks.html
Mock, Jillian. "Is Herd Immunity Our Best Weapon against COVID-19?" Discover, 4 May 2020, www.discovermagazine.com/health/is-herd-immunity-our-best-weapon-against-covid-19. Accessed 8 June 2020.
"Vaccines Protect Your Community." Vaccines.gov, US Dept. of Health and Human Services, Dec. 2017, www.vaccines.gov/basics/work/protection/index.html. Accessed 22 Feb. 2018.
"What Is Herd Immunity?" Vaccines Today. Vaccines Europe, 7 Feb. 2015, www.vaccinestoday.eu/vaccines/what-is-herd-immunity/. Accessed 22 Feb. 2018.
Willingham, Emily, and Laura Helft. "What Is Herd Immunity?" NOVA, PBS, 5 Sept. 2014, www.pbs.org/wgbh/nova/body/herd-immunity.html. Accessed 22 Feb. 2018.