Herd Immunity
In the simulations on this page, each circle represents one person in a community. When someone is infected with a virus, they can pass it to others by bumping into them.
What Is Herd Immunity?
Herd immunity is a simple idea. If enough people in a group are immune, a virus can’t spread. And if a virus can’t spread, it dies out. Poof, no more virus. No more sick people.
For a group to have herd immunity, a certain percentage of people in the group need to be immune. This is the herd immunity threshold. Immunity in the group on the right starts above the threshold, so fewer people get sick.
But—actually reaching herd immunity is not so simple. And there’s no one-size-fits-all way to do it.
Herd immunity applies only to infections that spread from person to person. If people get infected from an animal (like rabies) or direct contact (like tetanus), anyone who is not immune may get sick.
“Natural” Infection Is Not Enough
Many viruses are “one and done.” This means that you get the virus once, then you’re immune for life. But letting a virus infect a lot of people (spread “naturally”) doesn’t work for reaching herd immunity. It’s also not the kindest approach.
Just look at history. Vaccines are a recent invention. Before vaccination, the only way to become immune to a virus was to get infected and live through it. Yet, for as long as there have been people, viruses have continued to infect us. Smallpox, measles, polio, mumps. Over hundreds or even thousands of years, immunity through natural infection didn’t make them go away. Viruses have ways to survive even in populations where lots of people have immunity.
A huge factor here is that every day more people are born. These are people who have no immunity, who can be infected. And that’s one reason why, in the days before vaccination, sadly so many children died. Together with sanitation and antibiotics, vaccination changed what childhood looks like.
Natural infection causes sickness and death. And it simply doesn’t give a group a high enough level of immunity to stop a virus from spreading. Plus, some viruses, like HIV, stay with you for life. You can’t ever get rid of the infection and develop immunity.
Here, new people are being added to represent births. Because the new people are susceptible, the infection keeps spreading.
“Novel” viruses, like SARS-CoV-2, can be very dangerous. That’s because almost no one has immunity. Within months, this virus spread around the globe. It stressed healthcare systems and caused widespread suffering. To learn more, visit Why Are Novel Viruses a Big Deal?
Vaccination Is Key to Reaching Herd Immunity
Vaccination is a kinder way to give people immunity. It protects people without causing an infection. And it can reach more people in a group. It is key to reaching the herd immunity threshold. It also protects individuals who receive the vaccine even when the group is below the herd immunity threshold.
People who are vaccinated are protected from infection. Vaccination is also a way to bring a group over the herd immunity threshold—no natural infection needed. That’s how we completely got rid of smallpox.
Vaccination Protects Vulnerable Groups
When you get vaccinated, you protect the people who can’t.
Compared to infection, vaccination has two huge benefits. One, it is much safer. And two, it leads to less virus spreading around. That means people who do not have immunity are less likely to be exposed to the virus.
See, not everyone can gain immunity through vaccination. Due to their age or a medical condition, some people can’t take vaccines—like tiny babies, people who have gotten organ transplants, and people being treated for cancer.
That’s why everyone who can should get the vaccine. By getting vaccinated, you help to protect the people who can’t.
Calculating the Herd Immunity Threshold
As it turns out, different viruses have different herd immunity thresholds. The good news is there are ways to figure out the threshold. For that we can thank everyone who is out there tracking infections and researching viruses.
The biggest factor is how contagious a virus is—how easily an infected person can pass the virus to others. This is affected by things like how long someone can infect others before they have symptoms themselves and how the virus spreads. For example, a virus that spreads through the air transmits more easily than one that spreads through direct contact.
Scientists use a value called R0 (R-naught) to describe a virus’s transmission rate. The estimated R0 for measles ranges from 12 to 18. That means, in a group where no one is immune, one infected person on average will infect 12 to 18 others. It’s about 3 for smallpox. And for the flu, it’s just 1.3.
If a virus is highly contagious, like measles, the threshold for herd immunity is high. For less-contagious viruses, the threshold is lower.
Because measles is highly contagious (R0>=12), the herd immunity threshold is high—92% or more. For smallpox (R0=3), the herd immunity threshold is about 75%.
It also matters how long immunity lasts—what experts call durability. After some infections, immunity goes away over time. This can happen with vaccines too. Protection from mumps and yellow fever vaccines, for example, can start to fade after a few years. That means more people need to be vaccinated to reach herd immunity. If a vaccine lasts for life, a group can be protected even when the vaccination rate is lower.
A vaccine’s efficacy rate (how well it works) matters too. That’s the percentage of people who get full protection from the vaccine. No vaccine is 100% effective, but higher is better for reaching herd immunity.
Group Dynamics Affect Transmission Rates
In the early months of the COVID-19 pandemic, some countries slowed the virus’s spread, even though the immunity rate was near zero. They did it through physical distancing, mask wearing, and other measures that kept infected people away from healthy ones. By changing the way they interacted with one another, people decreased the virus’s spread in their communities. Measures like these can protect everyone, even when a group’s immunity is below the threshold.
By changing the way we interact, we can change a virus’s transmission rate.
Population density also affects transmission rates. People living close together in cities have more daily contacts than people living in rural areas. That means viruses usually spread more easily in cities.
Another important factor is that people don’t mix evenly or randomly. Sometimes they cluster. This can happen if a community lacks access to vaccines, or if people who avoid vaccines form communities. Groups like these are vulnerable because if one person gets infected, they can easily spread it to the others.
Putting it all together
By now you’ve seen that reaching herd immunity isn’t simple at all. This final simulation combines all the factors you’ve learned about in one place—and it’s still not as complex as real life! Use the blue bars to adjust any factors you’d like, and see how it plays out.
References
Anderson, R. M. (2016). The impact of vaccination on the epidemiology of infectious diseases. In The Vaccine Book (2nd edition), pp. 3-31. Bloom, B. R., & Lambert, P.-H., Eds. San Diego: Elsevier Science & Technology.
Cohen, J. (2019). How long do vaccines last? The surprising answers may help protect people longer. Science, 10.
Kennedy, R. B., Ovsyannikova, I. G., Thomas, A., Larrabee, B. R., Rubin, S., & Poland, G. A. (2019). Differential durability of immune responses to measles and mumps following MMR vaccination. Vaccine, 37(13), 1775-1784.
Lightning: Victim Data. Centers for Disease Control and Prevention. Accessed November 20, 2020.
Mallory, M. L., Lindesmith, L. C., & Baric, R. S. (2018). Vaccination-induced herd immunity: successes and challenges. Journal of Allergy and Clinical Immunology, 142(1), 64-66.
Randolph, H. E., & Barreiro, L. B. (2020). Herd Immunity: Understanding COVID-19. Immunity, 52(5), 737-741.
Vellozzi, C., Iqbal, S., & Broder, K. (2014). Guillain-Barre syndrome, influenza, and influenza vaccination: the epidemiologic evidence. CLINICAL INFECTIOUS DISEASES, 58(8), 1149-1155.