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Herd immunity occurs when enough people are immune to an infectious disease — through vaccination and/or prior illness — to make its spread from person to person unlikely.
It does not require 100% of the population to be vaccinated.
Throughout the COVID-19 pandemic, the term "herd immunity" has been tossed around as a future benchmark of progress in the fight against the virus. For over a year, health officials and news organizations have analyzed what herd immunity means for COVID-19, and whether and when the United States might achieve it.
But we recently came across a post on Instagram that makes a series of problematic claims about the effectiveness of vaccines and the concept of herd immunity.
Notably, it asserts that the benchmark revolves only around vaccines and can’t realistically be achieved because, "100 percent of the population would have to be vaccinated for that to work."
The post was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.)
This post is dubious for a couple of reasons. First, herd immunity does not depend on 100% of the U.S. population getting vaccinated. Everyone is encouraged to get the shot, because the more who do, the more likely it is that herd immunity can be achieved without putting too many more lives at risk.
Second, herd immunity doesn’t mean that no one gets the disease. It means that if enough people get the vaccine or have some natural immunity from a previous infection, the virus’ spread can be contained.
The post bases its claim on the premise that "today we take ‘herd immunity’ to mean that no one gets the disease if we all vaccinate."
That’s not the prevailing definition of herd immunity.
Herd immunity occurs when enough people are immune — either through vaccination or previous infection — to make its spread from person to person unlikely. That’s because an infected person is less likely to encounter a non-immune person to pass it on to, breaking that link in the chain of transmission.
When that happens on a large enough scale, the spread of the disease remains under control, though it’s not necessarily eradicated.
"Herd immunity is not ‘everybody is vaccinated therefore the disease goes away,’" said Dr. Sarah Fortune, chair of the Immunology and Infectious Diseases Department at the Harvard T.H. Chan School of Public Health.
"Herd immunity is, in fact, that enough people are vaccinated that the transmission chains are disrupted and people who are vaccinated, or not, are less likely to be exposed."
For COVID-19, scientists have tossed around different estimates of how much of the population needs to be immune, either naturally or by vaccine, in order for herd immunity to work well. The mathematical model for SARS-CoV-2 — which is derived based on how contagious the virus is and how the population behaves — puts it around 70 to 85 people out of every 100.
Even the most highly contagious viruses in the world don’t require 100% protection in a population to get them under control.
Take measles, for example. It’s considered the most contagious airborne virus that scientists are aware of, and medical experts say about 95% of the population needs to have protection for its circulation to remain low.
"Before the vaccine was available for measles, 90% of people got measles by the time they were 15," Fortune said, adding: "Measles isn’t gone, but the fact that a child now isn’t expected to be regularly exposed to it shows that herd immunity works."
In arguing that herd immunity can’t be achieved with vaccines, the Instagram post points out that vaccines don’t guarantee immunity. But the new COVID-19 vaccines’ high efficacy numbers undercut this point.
Clinical trials and follow-on studies show that the vast majority of people who get the COVID-19 vaccines are protected.
"The only people we found that don't produce an antibody response have highly dysfunctional immune systems," said Dr. Stanley Perlman, a professor of microbiology and immunology at the University of Iowa
A study by the Centers for Disease Control and Prevention found that the vaccines produced by Moderna and Pfizer-BioNTech appeared to be 90% effective in preventing COVID-19 infection in a real-world setting (outside of clinical trials).
And all three vaccines approved for emergency use in the U.S. have been proven to be highly effective at protecting against serious COVID-19 disease and death among those who do get infected.
An Instagram post claims that vaccine herd immunity doesn’t exist because 100% of the population would need to get vaccinated for that to be achieved.
The post misinterprets the meaning of herd immunity.
Herd immunity occurs when enough people are immune — either through vaccination or previous infection — to make its spread from person to person unlikely. It doesn’t mean completely eradicating the disease through vaccination, and it does not require 100% of the population to be vaccinated.
We rate this False.
Instagram post, April 25, 2021
PolitiFact, What we know about COVID-19 and immunity, Oct. 9, 2020
Mayo Clinic, Herd immunity and COVID-19 (coronavirus): What you need to know, April 22, 2021
Centers for Disease Control and Prevention, Vaccines and Immunizations Glossary, Updated July 30, 2020
Centers for Disease Control and Prevention, Interim Estimates of Vaccine Effectiveness of BNT162b2 and mRNA-1273 COVID-19 Vaccines in Preventing SARS-CoV-2 Infection Among Health Care Personnel, First Responders, and Other Essential and Frontline Workers, March 29, 2021
Kaiser Health News, Can Vaccination and Infection Rates Add Up to Reach Covid Herd Immunity?, March 17, 2021
Phone Interview, Dr. Stanley Perlman, professor of microbiology and immunology at the University of Iowa, April 28, 2021
Phone Interview, Dr. Sarah Fortune, Chair of the Department of Immunology and Infectious Diseases at Harvard T.H. Chan School of Public Health, April 28, 2021
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