What is Herd Immunity and When Will We Achieve It with COVID?

Wellness

Before March 2020, the term “herd immunity” would not have been found regularly in newspapers, and there would not have been podcasts on vaccine timetables and reports on who will or will not receive vaccines. That all changed upon our introduction to the COVID-19 pandemic. Since December 2019, COVID-19 has claimed millions of lives. Experts and healthcare officials point to herd immunity as our only way back to normalcy. Oxford Dictionary defines herd immunity as “the resistance to the spread of an infectious disease within a population that is based on pre-existing immunity of a high proportion of individuals as a result of previous infection or vaccination.” In simpler terms, herd immunity means that enough of a population is immune to a disease so that it cannot spread. Herd immunity means that even if someone cannot get vaccinated, they have protection. It protects everyone, but especially the vulnerable. If 80% of a population is immune to a specific disease, four out of five people who encounter someone who is sick with that specific disease will not become infected. Herd immunity is also a key concept for epidemic control. It acts as a barrier to disease. Immune people break the chain of transmission, so people who are not immune do not become sick. Johns Hopkins University has described the principle of herd immunity as “Herd immunity has to do with the protection of populations from infection which is brought about by the presence of immune individuals.” Vaccination is widely acknowledged as the safest way to reach herd immunity for various diseases, although some argue that herd immunity through natural immunity is the most effective way. Herd immunity is a term that has been in the news a lot in the past year, with the spread of COVID-19, as herd immunity is often pointed to as the only way for life to go back to normal. In this article I will discuss the history of herd immunity, how herd immunity happens, who benefits the most from herd immunity, and how/when we could achieve herd immunity with COVID-19.

History of herd immunity and pandemics

The term “herd immunity” originated in 1923. It was first published in a paper by Topley and Wilson titled “The Spread of Bacterial Infection: The Problem of Herd Immunity”. Herd immunity became a more popular term in the 1980s and 1990s, with the smallpox eradication and more focus on vaccinations for measles, tetanus, and the like. With the spread of COVID-19 in 2020 to the present day, there has been much focus on herd immunity and its potential to bring about the end of the pandemic and a return to normalcy. Many notable pandemics have been solved with herd immunity, such as measles. When it came to measles, the idea behind natural immunity was that if everyone just had it and got it over with, they’d be protected in the future, and the disease would go away. This approach is called herd immunity through natural immunity, and while it admittedly works, it comes at great loss of life. Natural immunity generally causes a stronger immune response, and sometimes causes lifelong immunity, but brings the risk of infection and death. It is for this reason that scientists and health experts generally hope to reach herd immunity through vaccination. Before vaccines were invented many years ago, natural immunity was the only way out of a pandemic. And as a result, scores of people died. While natural immunity has worked with other pandemics, it comes at great loss of life, and if there is a different option, such as vaccinating, that should be the clear decision. Some examples of pandemics that were solved with herd immunity through natural immunity are the Plague of Justinian and the Black Death. The Plague of Justinian spread across the Byzantine Empire in 541 CE, killing 30 to 50 million people, around half the world’s population at the time. The Black Death, which hit Europe in 1347, killing 200 million people, brought the concept of quarantining the sick. People had no scientific knowledge of how contagion worked, but they understood that it had something to do with proximity. Some officials in the port city of Ragusa decided to keep sailors on board their ship until they could prove they were not sick. At first, the sailors were held on their ship for 30 days, which was called trentio. Later, the period was increased to 40 days, called quarantino, which eventually evolved in Western culture to quarantine.

How the herd becomes immune

Herd immunity is usually reached by a combination of immunity through infection and immunity through vaccination. However, immunity through vaccination is by far the safer method, since immunity through infection or natural immunity, generally comes at a great cost of life. Immunity through vaccination, the preferred method of reaching herd immunity, is achieved when the “herd immunity threshold” is hit. The herd immunity threshold is the percent of the population needed to be immune to achieve herd immunity. In general, the threshold is about 70-90% of the population, but the percentage needed to achieve herd immunity becomes greater the more contagious the disease is. Herd immunity has different timelines depending on the disease. Again, for the more contagious diseases, it will take longer. And it depends on how quickly the population is becoming vaccinated, and how many people are reaching immunity through infection. Because of all the variables, there is no universal timeline for herd immunity for diseases in general, and even with individual diseases, as data and the disease evolves, the timeline can change.

Is it possible to reach herd immunity without a vaccine? Technically, it is possible to reach herd immunity without a vaccine. As has been stated before, herd immunity is achieved through a combination of immunity through infection and immunity through vaccination. So when herd immunity through vaccination is not an option, herd immunity is still attainable through natural immunity. However, herd immunity through natural immunity means many lives are lost, as the whole premise is to have everyone become sick, and not everyone’s bodies will be able to handle the disease. So whenever herd immunity through vaccination is available (and in modern times, it pretty much always is), that is always the preferred method. As seen in the above paragraph, vaccines are the preferred method of herd immunity. Reaching herd immunity through vaccination is more favorable and saves many more lives, rather than reaching herd immunity through natural immunity. According to the Centers for Disease Control and Protection, a vaccine is “a product that simulates a person’s immune system to produce immunity to a specific disease, protecting that person from that disease.” The goal of a vaccine is that the body that the vaccine is given mimics the infection on a smaller scale. If you have a vaccine for a specific disease, the chances of you getting that disease or spreading that disease fall drastically, depending on the disease. Vaccination protection can wane over time, which is why many vaccines have booster shots that are administered periodically. Measles, mumps, and chickenpox are all examples of diseases that were controlled by herd immunity through vaccination. Over recent years, with the rise of individuals who are concerned enough about the impact of vaccines on their bodies that they refuse all vaccines, the country has seen a resurgence of measles in communities. Diseases resurface in communities when those communities stop receiving vaccines, which means they lose their protection of herd immunity.

Who benefits the most from herd immunity? Those who benefit the most from a community reaching herd immunity are the vulnerable and those who cannot receive vaccines. The vulnerable would be those at higher risk of dying from any particular disease because of age or compromised immune systems, as well as babies and the unborn, who are too young to receive the vaccine. When we have herd immunity, our vulnerable and unvaccinated can safely live their lives, confident that those around them do not have a dangerous virus.

Herd immunity and COVID-19

COVID-19 is defined by Oxford Languages as “an acute respiratory illness in humans capable of producing severe symptoms and in some cases death, especially in older people and those with underlying health conditions. It was originally identified in China in 2019 and became pandemic in 2020.” At the time of writing, society has been living with the COVID-19 pandemic for over a year. Since it first surfaced in December 2019, it has claimed, as of April 1st, 2021, 2.82 million lives, 552,000 just in the United States alone. According to the Centers for Disease Control and Prevention, “The age-adjusted death rate increased by 15.9% in 2020. COVID-19 was the third leading cause of death.” Additionally, more than 3 million people died in the USA in 2020, making it the deadliest year in US history. For comparison, in 2019, 2,854,838 Americans died. Many experts say we will not be rid of COVID-19 completely in the near future, that more likely than not it’ll become endemic, as opposed to the epidemic it currently is. Per the Centers for Disease Prevention and Control, an endemic is the “constant presence and/or usual prevalence of a disease within a population in a certain geographic area.” The difference between an epidemic and an endemic is that an epidemic is a sudden increase of the disease, more cases than usual occurring. It comes from Latin; “epi” means upon, and “demic” means people, so an epidemic is something out among the people. As of March 2021, COVID-19 is currently an epidemic. Endemic is just out there; not doing anything, not many cases. Measles would be an example of endemic. So the consensus among experts is that COVID-19 will become endemic. “I think if you speak with most epidemiologists and most public health workers, they would say they believe this disease will become endemic,” said World Health Organization doctor David Heymann, speaking on COVID-19. Herd immunity is often pointed to as the long-awaited end of the pandemic. But reaching herd immunity for COVID-19 may not be all that simple. Some large factors include the willingness of the population to receive vaccinations, variants that may or may not become dominant, how long natural immunity lasts, the race to vaccinate, and vaccines not being effective against variants.

Firstly, the factor of the willingness of the population to receive vaccines. As has been stated before, the safest way to reach herd immunity is through vaccination. Dr. Ashley Drews, Assistant Professor of Clinical Medicine, Academic Institute at Houston Methodist and Weill Cornell Medical College, stated that “What we’ve needed since the start of this pandemic is a way to achieve wide-scale herd immunity without people ever being at risk for developing a severe infection.” Different diseases have different thresholds for herd immunity, but most experts estimate that COVID-19’s herd immunity threshold is somewhere between 80-90% of the population. According to Doctor Pilar Bradshaw, “We believe, based on current science-which, of course, is always changing- that about 85% of the population will need to be immune to COVID-19 in order for us to have adequate herd immunity.” If enough people do not receive the vaccine, we won’t be able to reach herd immunity. It’s estimated that at least 248 million Americans will need to be vaccinated so that the USA may reach herd immunity. In January 2021, only 50% of American adults said they would be willing to receive a COVID-19 vaccine, and in February, 40% were undecided and 15% said they’d never receive it. Only 80% of the USA population is eligible to receive the vaccine, so every eligible person needs to receive it. Additionally, kids under 16 (20 percent of the population) won’t be able to receive the vaccine for some time, so adults need to overcome their hesitancy. “There’s still a portion of the population that doesn’t want to take the vaccine and that’s working against us,” said Jeffery Shaman, Professor at Columbia University School of Public Health. So it is crucial that every person eligible to receive the vaccine receives it.

Secondly, another factor in if and when we reach herd immunity is whether variants become dominant. Variants are defined by Oxford Dictionary as “A form or version of something that differs in some respect from other forms of the same thing or from a standard.” COVID-19 has shown it mutates quickly, and many countries have experienced spikes in cases due to variants. Most healthcare works say the variants are a huge wildcard in the path to achieving herd immunity. Variants are a reason for alarm because we do not know if immunity, either from vaccination or infection, remains effective against the variants. If a new variant becomes dominant, the threshold for herd immunity could go up, and it would be hard for vaccinations to keep up, causing even more precautions to be needed. Dr. Anthony Fauci said in February that the USA isn’t vaccinating fast enough to keep ahead of the new variants. There have been plans to make vaccines for the new variants, and it appears that Pfizer’s vaccine is effective against variants, but nothing is definite. We just don’t know yet if vaccines and natural immunity are effective against the variants, so it is still important to keep wearing masks and social distancing. New variants could create a new wave of infections, even in people who have been vaccinated or previously exposed. In short, a new mutation could cause us to have to start all over again on the path to achieving herd immunity.

Thirdly, there’s the factor of how long natural immunity lasts. As COVID-19 is still a relatively new disease, little is known about the long-term effects, or how long immunity after infection lasts. It could differ from variant to variant, or even from person to person. It’s estimated that around 105 million Americans have been infected, but we don’t know what portion of that number is still immune. Until we know more about COVID-19 natural immunity, it’s impossible to know how much of the population is immune through infection.

Fourthly, the race to vaccinate. It’s crucial to vaccinate as fast as possible, because the faster we vaccinate, the faster we achieve herd immunity, and the more lives that are saved. Said Doctor Uzma Syid, infectious disease specialist, “That’s [herd immunity] the only way we’ll get this behind us. It’s essentially a race of us against this pandemic. And with more and more variants discovered, the more the urgency there is in getting people vaccinated.” Additionally, it’s important to vaccinate quickly to minimize the risk of another variant becoming dominant. Said Dr. Bradshaw, “The only way to accomplish that [herd immunity] is either by having many, many, more infections and deaths, which is not desirable, or to vaccinate with the COVID-19 vaccine.” But even in most scenarios, millions of people will be infected and thousands more will die before we reach herd immunity. Let’s take a look at the most likely scenarios. (all scenarios are based on starting March 1st and the assumption that an average of 3 million vaccines is given each day.) In scenario 1, we end all restrictions, (mask-wearing, social distancing, etc) in April, when around 15% of the population is vaccinated. In that scenario, we reach herd immunity in June, but 170,000 lives are lost. In scenario 2, we keep restrictions in place until most people are vaccinated. Here, we reach herd immunity in July and lose 100,000 lives. In scenario 3, we end all restrictions in March. We reach herd immunity in May, but in this scenario, 320,000 deaths occur. And that’s all before you factor in the threat of more contagious variants becoming dominant.

Finally, there’s the question of whether vaccines are effective against variants. So far, it has appeared that most vaccines at least minimize the risk of infection from other variants, but COVID-19 has shown the ability to mutate quickly. So it’s important to reach herd immunity as quickly as possible, to eliminate the risk of a variant that our vaccines are not effective against becoming dominant.

Once we reach herd immunity with COVID-19, to maintain it we will need to continue to vaccinate to protect against it. As experts have said, COVID-19 will most likely become endemic, so if a community has a low vaccination rate, it would be very possible for an outbreak to occur. And until we do reach herd immunity, it is incredibly important to continue wearing masks and social distancing. Dr. Ashley Drews stated “The fact is that, right now, your squad isn’t just your five friends. Your squad is also the three people in someone else’s crew, the ten people in another person’s pod, as well as your family and their own squads, crews, and pods. It’s also your coworkers, your kid's teachers, and the random people you encounter running errands. There are no squads, crews, and pods during this pandemic, only the herd.” However, it does seem likely that the worst of the pandemic is over. Dr. Anthony Fauci said in February that “the best-case scenario for me is we get 85% of people vaccinated by the end of the summer. If we do that by the time we get to fall, we can approach a degree of normalcy.

There has been controversy over the best way to reach herd immunity with COVID-19. Some argue that herd immunity through natural immunity is the way to reach the end of the pandemic, as opposed to herd immunity through vaccination. However, the World Health Organization and the Centers for Disease Control and Prevention have stated countless times that herd immunity through vaccination is the safest way out. According to World Health Organization Director-General Tedros Ghebreyesus, “Allowing a dangerous virus that we don’t fully understand to run free is simply unethical.” Herd immunity through natural immunity is very dangerous, especially with a highly contagious disease like COVID-19. Sweden reached herd immunity through natural immunity, and it was called dangerous by health leaders globally. As COVID-19 is still a new disease and little is known about it or its long-term effects, getting it to get it over with is a terrible idea. While natural immunity has worked with other pandemics, it came at great loss of life, and if there’s a different option, such as vaccinating, that should be the clear decision.

In conclusion

Herd immunity is when enough people in a population are immune to a disease to prevent its spread, and herd immunity through vaccination is the safest and most effective way to achieve herd immunity. When it comes to herd immunity and COVID-19, some key factors in if and when we achieve herd immunity are the willingness of the population to get vaccinated, the spread of variants, and the effectiveness of the vaccines. It’s incredibly important to have every eligible person vaccinated because we have to reach the herd immunity threshold. One of the most important takeaways from this research paper would be the importance of vaccines in achieving and maintaining herd immunity. Without vaccines, our only option for reaching herd immunity would be through natural immunity. Herd immunity is a key concept in protecting communities and the vulnerable to diseases.

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Caroline Rendell
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Caroline Rendell is a rising 10th grader in West Chester, Pennsylvania, where she attends Rustin High School. She joined TTM in August 2020 and when not tackling the workload known as high school, can be found baking, bullet journaling, watching sports, hanging out with friends, or procrastinating for hours by looking at memes.