What Is Herd Immunity? Clarifying Common Misconceptions
By Ashley Brooks on 03/22/2021
Herd immunity is a concept that’s been around for decades, but the COVID-19 pandemic has launched it into buzzword territory. You’ve heard a lot about the topic of herd immunity since 2020, especially in relation to vaccines. Given the pandemic’s far-reaching impact, it seems as though everyone and their dog has taken up an amateur interest in epidemiology. With that comes with a lot of different—and loud—opinions on this hot-button public health issue, and you’re not sure how to separate fact from fiction.
So what is herd immunity? Like many complex medical concepts, some of the nuance of herd immunity can get lost in translation. But your understanding of herd immunity isn’t something to leave in foggy territory. People’s beliefs about herd immunity can influence their behaviors and choices in a way that impacts the trajectory of the COVID-19 pandemic—or the spread of other infectious diseases, so it’s important to stay informed.
We’re setting the record straight and clearing up common questions and misconceptions with the help of public health expert Dr. Joyvina Evans, professor at the Rasmussen University School of Health Sciences. Join us as we gain answers to questions like “How does herd immunity work?” and more.
What is herd immunity?
Let’s start with some basic definitions. Although you might hear people on the news or social media throw around the words “herd immunity” with differing meanings, the medical community agrees upon one standard definition.
“In simple terms, herd immunity is a form of protection that occurs when people in a community become immune to a disease,” Dr. Evans says. “Herd immunity protects the individual and everyone else in the community.”
The basic idea is that if enough people in a single community, or “herd,” have the antibodies needed to protect against a particular disease, that disease will have a hard time spreading from person to person. This is important because some people in the community are especially susceptible to diseases, such as babies and young children, the elderly and those with compromised immune systems.
How to achieve herd immunity
Herd immunity as a concept is simple, but in practice, it gets complicated. “Herd immunity depends on the contagiousness of the disease. Diseases that spread easily, such as measles, require a higher number of immune individuals in a community to reach herd immunity,” according to the Association for Professionals in Infection Control and Epidemiology (APIC).
There are only two ways for a community to reach herd immunity: through the natural development of antibodies or vaccination.
The immune system naturally develops resistance to a disease after being infected, although “immunity” occurs at varying levels depending upon the individual and the disease. This is how people resisted disease for centuries before vaccines began to be consistently developed in the 1800s and widely distributed in the 1900s.1
There was just one major catch: you had to survive the disease first. With death rates varying from around 30 percent for smallpox and polio to 0.3 percent for measles, achieving natural herd immunity could be a weighty gamble with your life.2, 3, 4
That’s where vaccines come in. Vaccines allow people to develop antibodies to diseases without exposing them to the disease itself. Not only does this reduce the threat of death or other complications that can occur with natural herd immunity, but it also allows researchers to control the level of immunity a person typically develops—a factor that can vary quite a bit if people are infected by a disease “in the wild.”
The factors that affect herd immunity
Unfortunately, herd immunity isn’t foolproof. “The success of herd immunity depends on the number of people who become immune to the disease,” Evans says. This may seem like a no-brainer, but there are several factors that can influence immunity.
The level of antibodies a person develops after becoming sick with a disease can vary, with some people mounting a stronger immune response than others. Some vaccinations and infections, like pertussis (whooping cough), don’t convey lifetime immunity, which is why booster shots are needed.5 Others, like COVID-19, are still being studied to see how immunity levels may wane over time.
Because of these unknowns in natural herd immunity, vaccination is the simplest way to control a community’s protection against disease. “Herd immunity fails if too few people are vaccinated,” Evans says.
People choose not to be vaccinated for many reasons, including religious beliefs, skepticism, allergies and underlying health conditions. “One of the drawbacks of herd immunity is that people who have the same beliefs about vaccinations” often group themselves into the same communities, leading to “potentially large groups of unvaccinated people close together,” according to APIC. This leaves room for community outbreak and is one of the leading factors that can cause herd immunity to fail.
Common misconceptions about herd immunity
Now that you have a solid foundation of what herd immunity is, let’s talk about what herd immunity isn’t. There’s plenty of misinformation about this topic, so it’s important to be aware and educated about these herd immunity myths.
Myth #1: It’s easy to achieve natural herd immunity without vaccination.
Thanks to the different factors that impact the effectiveness of herd immunity, achieving natural herd immunity can be incredibly difficult if not impossible—as recurring epidemics of past centuries have taught us. Even if it can be achieved, it would likely come with a high cost.
According to Dr. Howard Forman of Yale University, achieving herd immunity to the COVID-19 virus in the United States without vaccinations would require 70 percent of the population to be infected with the disease. Reaching this rate of infection would likely leave an additional one million Americans dead.6
Myth #2: “Everyone else is getting vaccinated, so I don’t have to.”
“One of the most common misconceptions that I have heard is people believing that they do not need to obtain vaccination since everyone else is getting vaccinated,” Evans says. “The issue is what if the neighbors, cousins, parents choose not to receive the vaccination?”
Herd immunity only works if enough of the population is on board, either through previous infection or vaccination. It’s easy to assume that everyone else has antibodies, so it doesn’t matter if you receive a vaccine or not. But when enough people make this type of assumption, the community as a whole will struggle to reach the herd immunity threshold, leaving space for infectious disease to continue spreading through the population. Eventually, the disease may run out of transmission pathways among the unvaccinated, but vaccination is typically the safest way to close those routes.
Myth #3: Disease outbreaks will never happen once we reach herd immunity.
Reaching herd immunity unfortunately doesn’t mean we’re all invincible against disease. The only vaccine-preventable disease to be declared eradicated from the world is smallpox. Although disease outbreaks become increasingly rare upon reaching herd immunity, transmission can still occur as long as a disease exists in the world.
International travel from regions that still have high infection levels, as well as pockets of certain communities or populations that remain unvaccinated, can provide opportunities for outbreaks to occur even with herd immunity.
Public health workers: Helping us safely achieve herd immunity
What is herd immunity? Now you have a better understanding of this complex medical concept—and you also know that achieving herd immunity isn’t always easy. It’s up to public health workers to continue steering the country toward developing herd immunity against COVID-19 or maintaining it against other infectious diseases. Maybe you’re even thinking of joining their ranks yourself!
Public health is a large field with a variety of unique roles that all work together. Discover the roles available in the public health field by reading “What Can You Do with a Public Health Degree? Exploring Your Options.”
1Immunization Action Coalition, Vaccine Timeline [information accessed February 2021], https://www.immunize.org/timeline/
2Harvard Medical School, Harvard Health Publishing, Smallpox [information accessed February 2021], https://www.health.harvard.edu/a_to_z/smallpox-a-to-z
3Centers for Disease Control and Prevention (CDC), Epidemiology and Prevention of Vaccine-Preventable Diseases – Pink Book, Poliomyelitis [information accessed February 2021], https://www.cdc.gov/vaccines/pubs/pinkbook/downloads/polio.pdf
4Centers for Disease Control and Prevention (CDC), Complications of Measles [information accessed February 2021], https://www.cdc.gov/measles/symptoms/complications.html
5Pediatric Infectious Disease Journal, Duration of Immunity Against Pertussis after Natural Infection or Vaccination [information accessed February 2021], https://pubmed.ncbi.nlm.nih.gov/15876927/
6Yale News, “What Is Herd Immunity?” [information accessed February 2021], https://news.yale.edu/2021/01/06/what-herd-immunity