The history of herd immunity, how it is achieved, and its implications in the current pandemic
Written by: Shriya
Herd immunity is an important concept that has been prevalent in public health for over 100 years. With the current global pandemic, it has become a topic associated with much controversy. Before we look into the implications of herd immunity in our present day situation, it is important to learn about the history of the concept and what it actually presents.
A Brief History:
The phrase ‘herd immunity’ seems to have first appeared in the work of livestock veternarians who were concerned about contagion abortion, which is an epidemic of spontaneous miscarriage in cattle and sheep. This disease was vastly prevalent in the 1910s, when it became the leading contagious threat to cattle in the United States. In the Journal of the American Veterinary Medical Association, Kansas veterinarians George Potter, along with Adolph Eichhorn envisioned “herd immunity.” As he wrote in 1918, “Abortion disease may be likened to a fire, which, if new fuel is not constantly added, soon dies down. Herd immunity is developed, therefore, by retaining the immune cows, raising the calves, and avoiding the introduction of foreign cattle.” (1) Potter’s concept of herd immunity quickly reached the UK, where it was crucial in fighting against infections throughout World War I.
This idea then moved to be applied in medicine. In July, 1919, bacteriologist W. W. C. Topley described experimental epidemics he created in groups of mice. Unless there was a steady influx of susceptible mice, the rising prevalence of immune individuals would end an epidemic. In a 1923 article in the Journal of Hygiene, he and G. S. Wilson described this phenomenon as “herd immunity.” (1) In 1923 Sheldon Dudley, a professor of pathology, became aware of epidemics of diphtheria at the Royal Hospital School in Greenwich. The school provided laboratory-like conditions, with a group of male students, who entered in batches several times a year, where they slept in dormitories. Through this, Dudley applied “herd immunity” to humans.
Herd immunity became a fixture of epidemiology by the 1930s. Discussions of herd immunity for influenza, polio, smallpox, and typhoid appeared in textbooks, journals, and public health reports in England, Australia, and the USA. It took on fresh prominence in the 1950s and 1960s as new vaccines raised crucial questions for public health policy. (1) The idea surged again after 1990 as public health officials worked to achieve sufficient levels of vaccine coverage.
Now that we understand how the concept of herd immunity came to be, we can take a look at what it is and how it can be achieved.
What is Herd Immunity:
Herd immunity occurs when a high percentage of the community is immune to a disease, which makes the spread of this disease from person to person less likely. Even individuals that are not vaccinated are therefore offered some level of protection since the disease has little opportunity to spread within the community. As a result, not every single person needs to be vaccinated in order to be protected, which helps ensure vulnerable groups who cannot get vaccinated, such as babies, pregnant women, and immunocompromised people, are kept safe. For example, the earliest a baby can receive their first pertussis or whooping cough vaccine is at two months, and the earliest a child can receive their first measles vaccine is at one year, making them vulnerable to these diseases. (2)
Herd immunity depends on the contagiousness of the disease. This means that the percentage of people who need to have antibodies in order to achieve herd immunity against a particular disease varies with each disease. (3) Diseases that spread easily require a higher number of immune individuals in a community to reach herd immunity, while those that do not spread as easily can allow herd immunity with a lower number of immune individuals in the community. For example, herd immunity against measles requires about 95% of a population to be vaccinated. The remaining 5% will be protected by the fact that measles will not spread among those who are vaccinated. For polio, the threshold is about 80%. (3)
How is herd immunity achieved?
There are two ways to achieve herd immunity: vaccines and infection.
Vaccines
Vaccines create immunity without causing illness or resulting complications. Using the concept of herd immunity, vaccines have successfully controlled deadly contagious diseases such as smallpox, polio, diphtheria, rubella and many others. (4)
However, reaching herd immunity through vaccination sometimes has drawbacks. Protection from some vaccines can wane over time, requiring revaccination. Sometimes people may not get all of the shots that they need to be completely protected from a disease, which would prevent them from actually achieving immunity. Additionally, some people may object to vaccines because of religious objections, fears about the possible risks or skepticism about the benefits. People who object to vaccines often live in the same neighborhoods or attend the same religious services or schools. If the proportion of vaccinated people in a community falls below the herd immunity threshold, exposure to a contagious disease can result in the disease quickly spreading. (4) In this way, opposition to vaccines can pose a major challenge to herd immunity.
Natural infection
Herd immunity can also be reached when a sufficient number of people in the population have recovered from a disease and have developed antibodies against future infection. For example, those who survived the 1918 flu (influenza) pandemic were later immune to infection with the H1N1 flu, a subtype of influenza A. During the 2009-10 flu season, H1N1 caused the respiratory infection in humans that was commonly referred to as swine flu. (4)
As with vaccinations, natural infection as a means to reach herd immunity also has drawbacks. Research suggests that after infection with some viruses, reinfection with the same virus - though usually mild and only happening in a fraction of people — is still possible, whether that be after a period of months or years. This can pose a threat to achieving herd immunity.
COVID-19:
As many countries around the world recognised the magnitude of the COVID-19 pandemic in March, 2020, some seemed to put their faith in herd immunity. However, in August, WHO's Michael Ryan warned journalists “we are nowhere close to the levels of immunity required to stop this disease transmitting. We need to focus on what we can actually do now to suppress transmission and not live in hope of herd immunity being our salvation.” The appeal of herd immunity is easy to understand: if it is reached, an epidemic ends. But the illness and death that such an approach would require has prompted a strong backlash. (5) Commentators in The Lancet concluded that “In light of these findings, any proposed approach to achieve herd immunity through natural infection is not only highly unethical, but also unachievable” (5).
Even if infection with the COVID-19 virus creates long-lasting immunity, a large number of people would have to become infected to reach the herd immunity threshold, which is far too risky. Experts estimate that in the U.S., 70% of the population — more than 200 million people — would have to recover from COVID-19 to halt the epidemic. (4) However, attempts to reach ‘herd immunity’ through exposing people to a virus are scientifically problematic and unethical. Letting COVID-19 spread through populations will lead to increased infections, suffering, and death, since the vast majority of people in most countries remain susceptible to this virus. This could not only lead to serious complications and millions of deaths, but would also further overwhelm the healthcare system. Additionally, there is still much that is unknown about immunity to COVID-19. Most people who are infected develop an immune response within the first few weeks, but it is not known how strong or lasting that immune response is or how it differs between people, since there have been reports of individuals being infected a second time. Until we better understand COVID-19 immunity, it will not be possible to know how much of a population is immune and how long that immunity lasts for, let alone make future predictions. These challenges should preclude any plans that try to increase immunity within a population by allowing people to get infected. (5) Therefore, public health experts believe that herd immunity would not be an ethical means to combatting the pandemic.
Though currently surrounded by controversy, herd immunity remains an important concept in public health. It is a way to not only reduce the prevalence of diseases, but also protect those who are unable to receive the vaccines necessary to protect themselves, allowing for our community to remain safe and healthy.
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