Herd immunity—sometimes called community immunity—is like a shield that protects everyone in a group, even those who can’t protect themselves. This idea has shaped how we fight diseases for over a century. Let’s explore its history, science, and real-world impact.
History and Background
The begins with animals. In the early 1900s, scientists studying livestock noticed that when most animals in a herd survived a disease, the whole group became safer. This observation led to the term “herd immunity.” By the 1920s, , showing that diseases couldn’t spread easily if enough mice were immune.
Later, doctors realized this concept applied to humans too. For example, when many people in a community recovered from measles or polio, outbreaks became smaller. But relying on natural infections was risky—many people died or suffered lifelong disabilities. Vaccines changed everything. Starting with smallpox in the 18th century, , making herd immunity safer and more effective.
Epidemiological Applications
Herd immunity works like a chain reaction. Imagine a virus as a fire: if too many people are “flammable” (susceptible), the fire spreads quickly. But if enough people are “fireproof” (immune), the flames die out. like newborns, elderly individuals, or people with weakened immune systems who can’t get vaccinated.
Diseases need non-immune people (or animals) to survive. They are known as susceptible hosts. When vaccination rates are high, the virus hits dead ends. For example, . If vaccination drops below this threshold, outbreaks return, as seen in recent measles outbreaks and .
Mathematical Foundations
So how do we know the approximate level of vaccination needed for herd immunity? Math.
The math behind herd immunity starts with a number called R₀ (pronounced “R-naught” and sometimes referred to as the “”). R₀ tells us how contagious a disease is. For example:
- Measles has an R₀ of about 15 (one person infects 15 others).
- COVID-19’s .
To calculate the herd immunity threshold, scientists use the formula: 1 – (1/R₀).
For measles (R₀=15), this means 1 – (1/15) = 1 - 0.067 = 0.933, or about 93% immunity needed. If fewer people are immune, the disease keeps spreading. For original COVID, herd immunity was achievable at around 66% immunity, but variants and people’s hesitance in getting vaccinated made it a difficult goal to achieve.
Key Assumptions
Herd immunity models rely on simplifications:
- : Everyone interacts equally. In reality, cities have closer contact than rural areas.
- Lasting immunity: Some diseases (like measles) grant lifelong protection; others (like COVID-19) weaken over time because of virus mutation or the type of immune response they trigger.
- No mutations: New variants can evade immunity, as seen with COVID-19’s Delta and Omicron strains, or with the yearly influenza viruses.
These assumptions mean real-world herd immunity is messier. For instance, a town may have immunity levels above 93% for measles, but a school full of unvaccinated children might have an immunity level of 10%. The school is then at risk of a severe pandemic, but it would be localized to the school and difficult to spread to the whole town.
Or a school can have every single child vaccinated against influenza, but a new strain not covered by the vaccine emerges and makes many students sick.
Applications in Vaccine Policy
. Global vaccination campaigns have eradicated smallpox from the planet, and they have eliminated polio from almost all countries in the world. (Only one type of the original three polioviruses remains, too.) Governments use herd immunity thresholds to set vaccination goals. For example:
- Measles: 95% vaccination rate.
- Polio: 80% vaccination rate.
Vaccine hesitancy disrupts these efforts. In 2018, two nurses in the Pacific island nation of Samoa used an anesthetic instead of saline to prepare vials of the measles vaccine. Unfortunately, children died from the accident. Parents became fearful of the vaccine because they were not informed about the accident, as the government kept quiet about what really happened. Then local and international anti-vaccine activists swooped in and amplified the fears. This resulted in a quick drop of vaccine coverage, far below herd immunity levels.
By the end of 2019, measles vaccine coverage was around 30%. As a result, . Hundreds were hospitalized. The government had to declare an emergency and mandate measles vaccination, in some cases forcefully.
Examples of Herd Immunity in Action
- Smallpox: Widespread vaccination led to its eradication in 1980—the only human disease eliminated this way. (Another disease, called Rinderpest, was .)
- Measles: Before vaccines, . By 2000, measles was eliminated in the United States, meaning it wasn’t a common childhood disease anymore. That is, until declining vaccinations caused new outbreaks.
But the rest of the countries in the American Continent remain protected. : “Despite the rise in cases in other regions, the Americas has not reported measles-related deaths, and it is estimated that between 2000 and 2022, the measles vaccine alone prevented 6 million deaths in the region.” Why? Because people in Canada and Latin America have resisted the misinformation on measles, and they continue to vaccinate their children at high rates. - COVID-19: Vaccines reduced hospitalizations, but uneven global vaccination and variants like Omicron delayed herd immunity. .
Conclusion
Herd immunity isn’t a magic number. It’s a balance between science, society, and policy. Vaccines remain our best tool to protect communities without the cost of widespread illness. However, achieving herd immunity requires trust in science, access to vaccines, and global cooperation. As history shows, when we work together, diseases like smallpox can vanish forever. But when we let our guard down, old threats return.
Resources and Additional Reading
- Fine P, Eames K, Heymann DL. “Herd immunity”: a rough guide. Clin Infect Dis. 2011;52(7):911-916. doi:10.1093/cid/cir007
- Anderson RM, May RM. Vaccination and herd immunity to infectious diseases. Nature. 1985;318(6044):323-329. doi:10.1038/318323a0
- World Health Organization. Coronavirus disease (COVID-19): Herd immunity, lockdowns and COVID-19. Published December 31, 2020.
- Randolph HE, Barreiro LB. Herd immunity: understanding COVID-19. Immunity. 2020;52(5):737-741. doi:10.1016/j.immuni.2020.04.012
- Oxford Vaccine Group. Herd Immunity: How does it work? Published January 20, 2025.
This article was created on February 23, 2025. There are no previous versions of this article.