Updated March 12, 2021
Tiny organisms like bacteria and viruses can cause infectious diseases, which can devastate communities.
As an infectious disease spreads in a community, it can lead to a high number of illnesses — and even death. The disease may continue to spread until enough people are immune to what’s causing it.
When enough people become immune, it can slow or stop the disease’s spread. That is known as herd immunity.
What Is Herd Immunity?
Herd immunity is the term for the situation that occurs when enough of a population becomes immune to a disease to stop its spread.
The people in the population — or the “herd” — can become immune to a disease through vaccination or by recovering from a previous infection.
“Herd immunity is when you have enough immunity in a population so that a disease stops spreading,” says John Goldman, MD, vice president, Medical Affairs, designated institutional official, and epidemiologist, UPMC Pinnacle.
“What happens is when you have people who are not immune, it’s easy for the virus to find a new host, to go from one person to another. When you have herd immunity, there are so many people that have already been exposed to the virus, or already have immunity through vaccination, that the virus can’t easily spread itself. It can’t find a susceptible host.”
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How Does Herd Immunity Work?
Herd immunity occurs when a significant amount of a community’s population becomes immune to a disease. When herd immunity is achieved, it becomes more difficult for a disease to spread from person to person.
“What happens with herd immunity is a pretty simple concept,” Dr. Goldman says. “The virus can’t find someone who can be infected.”
The percentage of the population that needs to be immune for herd immunity to be achieved varies, depending on how infectious the disease is.
With diseases that spread easily — like measles — a higher percentage of people need to be immune to reach herd immunity.
“If you have a disease that’s relatively noninfectious, you need fewer people in a population who are immune,” Dr. Goldman says. “If you have a disease that’s very infectious, you need a higher percentage of people in the population who are immune.”
In general, anywhere between 70% and 90% of the population needs to be immune to a disease for herd immunity to be achieved, according to a report in the Journal of the American Medical Association.
Vaccines are the most common way to reach herd immunity, although it also can be achieved through prior illness in a community.
Vaccines and Herd Immunity
Vaccines contain weakened or inactive germs — or portions of the germs — that cause disease. They stimulate your body’s immune system to create antibodies against the disease, making you immune.
With a vaccine, you can achieve immunity to a disease without ever becoming ill.
If a significant number of people in a given population get vaccinated against a disease, it can create herd immunity to that disease.
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Prior Illness and Herd Immunity
Your body’s immune system naturally creates antibodies to fight off disease when you are sick. The immune system can remember how to make these antibodies if you encounter the germ again.
It’s possible to reach herd immunity if enough people in a community become immune after getting exposed to a disease. The number of people it would take depends on the disease.
For diseases with low morbidity and mortality, like mononucleosis, herd immunity through natural infection can be reached more easily without a large effect on the population. People are frequently infected and frequently get sick but do not end up in the hospital or die.
But for diseases with a higher mortality rate, achieving herd immunity through prior illness requires that a large proportion of the population is infected, and there’s a higher risk of being hospitalized or dying.
Could Herd Immunity Protect Us?
In simple terms, herd immunity protects everyone. Achieving herd immunity means diseases can’t spread easily, making it more difficult for people to become sick.
Herd immunity through vaccination has helped make diseases like measles, polio, mumps, and chickenpox rare in the United States.
Herd immunity also can protect people who may not be able to get vaccinated.
Groups like older adults, people with weakened immune systems, and people who are allergic to some vaccine ingredients may not be able to get vaccinated — or a vaccine may not have as strong an effect for them.
If enough people in the community get vaccinated, the disease does not spread. This protects even non-immune people because herd immunity prevents the virus from spreading and protects the vulnerable from ever being exposed.
Herd Immunity and COVID-19
The COVID-19 pandemic has caused more than 100 million illnesses and millions of deaths worldwide since late 2019. In the United States alone, more than 500,000 people have died because of COVID-19.
It is estimated that 60 to 80% of the population would have to be immune to COVID-19 for herd immunity to be reached, Dr. Goldman says. But the exact number is unknown because the infection rate is not completely known.
Widespread vaccination is the best way to reach herd immunity against COVID-19. The Food and Drug Administration (FDA) has issued emergency use authorization to three COVID-19 vaccines: developed by Pfizer-BioNTech, Moderna, and Johnson & Johnson/Janssen.
The approved vaccines are all safe for use and effective against COVID-19, especially in preventing severe illness, hospitalization, and death.
It is likely that a significant amount of the population must get the vaccine for COVID-19 herd immunity to be established. In the U.S., that would equate to the widespread vaccination of hundreds of millions of people.
Can natural immunity protect us against COVID-19?
Herd immunity to COVID-19 through infection is not a good substitute for a vaccine. The death rate for COVID-19 is significantly higher than similar diseases like the flu.
Based on the current mortality rate, herd immunity to COVID-19 through infection would likely lead to a high number of illnesses and deaths.
“If we achieve herd immunity through actual infection with COVID, we can expect between 1 million and 1.5 million Americans to die,” Dr. Goldman says. “We would have a level of death that I think most Americans would find unacceptable.”
When Will Herd Immunity Against COVID-19 Be Reached?
Vaccine distribution is currently taking place throughout the United States. However, a significant amount of the population — anywhere between 60% and 90% of Americans — must get vaccinated to reach herd immunity.
Because more than 300 million people live in the United States, herd immunity will take several months.
Because of how quickly SARS-CoV-2 mutates, it is possible for variants of the virus to continue to circulate in the future, similar to the flu. But vaccines and other methods of immunity can keep it in check.
“Another possibility is that COVID is still with us, that we never achieve full herd immunity, but infection with COVID becomes more like infection with influenza,” Dr. Goldman says.
“We may eventually have both a flu season and a COVID season, probably at about the same time. You may eventually get both a yearly flu shot and a yearly COVID shot. Eventually, COVID may become ‘just the flu’ and we will not need the masks, the social distancing, the high rates of hospitalization, and the deaths.”
Vaccination is likely to get us to COVID-19, Dr. Goldman says. Until it does, it is important to keep wearing masks, social distancing, and protecting anyone who is at high risk for COVID-19.
For more information on COVID-19, visit UPMC.com/COVID19.
Association for Professionals in Infection Control and Epidemiology, Herd Immunity. https://apic.org/monthly_alerts/herd-immunity/
Centers for Disease Control and Prevention, Basics of Vaccines. https://www.cdc.gov/vaccines/vpd/vpd-vac-basics.html
Centers for Disease Control and Prevention, Vaccine Glossary. https://www.cdc.gov/vaccines/terms/glossary.html
Harvard Health Publishing, Preventing the Spread of the Coronavirus. https://www.health.harvard.edu/diseases-and-conditions/preventing-the-spread-of-the-coronavirus
Rita Rubin, MA, Journal of the American Medical Association, Difficult to Determine Herd Immunity Threshold for COVID-19. https://jamanetwork.com/journals/jama/fullarticle/2769704
U.S. Department of Health and Human Services, Vaccines, Vaccines Protect Your Community. https://www.vaccines.gov/basics/work/protection
World Health Organization, Coronavirus Disease (COVID-19) Dashboard. https://covid19.who.int/
About Infectious Diseases
If you have a disease caused by bacteria, fungi, parasite, or virus, the UPMC Center for Care of Infectious Diseases can help. We have specialty units for prevention and treatment of HIV-AIDs, postsurgical and transplant infections, and illnesses caused by international travel. Our faculty research infectious diseases and participate in clinical trials to learn more and develop better treatment and prevention methods.