Updated February 16, 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, or population immunity.
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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.
The percentage of the population that needs to be immune for herd immunity to be achieved varies depending on the disease and community. With diseases that spread easily, a higher percentage of people need to be immune to reach herd immunity.
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 teach your immune system what a germ looks like without causing the infection itself. They stimulate your body’s immune system to create antibodies against the disease, making you immune to it.
With a vaccine, you can achieve immunity to a disease without ever becoming infected.
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. Some of those antibodies may remain after you fight off the infection, which can provide a certain degree of immunity against future infection.
It’s possible reach herd immunity if enough people in a community become immune after getting sick. The number of people it would take depends on the disease.
Depending on the severity of the disease, however, achieving herd immunity through prior illness may lead to significant numbers of severe illness or deaths in a community.
Who Does Herd Immunity Protect?
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, it can protect vulnerable people from disease.
Herd Immunity and COVID-19
The COVID-19 pandemic has caused tens of millions of illnesses and more than a million deaths worldwide since late 2019. In the United States, hundreds of thousands of people have died because of COVID-19.
It is currently estimated that 60 to 70% of the population would have to be immune to COVID-19 for herd immunity to be reached, according to Harvard Health Publishing. But the exact number is unknown.
A vaccine is necessary for herd immunity to COVID-19 to be reached.
The U.S. Food and Drug Administration has authorized two COVID-19 vaccines as both safe and effective for wide human use, and more vaccines are being researched throughout the world. A significant amount of the population must get the vaccine for herd immunity to be established. Herd immunity to COVID-19 likely cannot happen without widespread vaccination.
Herd immunity to COVID-19 through infection with the disease is not a good substitute for a vaccine. The death rate for COVID-19 is significantly higher than similar diseases like the flu. Herd immunity to COVID-19 through infection would likely lead to a high number of illnesses and deaths.
While we wait for the COVID-19 vaccine to be distributed widely, we must continue to practice safety measures. That includes wearing cloth face coverings when in public, maintaining effective social distancing, and practicing good hand hygiene.
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.