What Is ‘Herd Immunity’ and Can It Stop COVID-19?
Widespread vaccination may help bring the pandemic to its knees, but we're not there yet.
A concept bandied about early in the coronavirus pandemic, something called "herd immunity," has surfaced again. This time people are talking about it in the context of the nation's expanding vaccination program. How long before a substantial swath of the population is protected from COVID-19?
Here’s what public health experts have to say about achieving herd immunity in America and when we can expect to resume life as we used to know it.
What is herd immunity, and how does it work?
Herd immunity (aka community immunity) occurs when enough people in a population are immune to a disease, making person-to-person transmission unlikely. The infection, in effect, no longer has the upper hand.
Herd immunity can be achieved in one of two ways: through vaccination or prior illness, says the Centers for Disease Control and Prevention (CDC).
Your immune system naturally develops infection-fighting antibodies when it encounters a disease. After you recover, you're left with some level of antibody protection against the illness. (Public health experts are quick to point out that allowing vast number of folks to fall ill to a highly infectious, potentially deadly disease is not an ethical pathway for achieving herd immunity. Plus, it's not clear that prior COVID-19 infection is protective against future infection, as Mayo Clinic notes.)
Vaccines, by contrast, work by training your immune system to create antibodies so that when you encounter the disease, your body will quickly mount a defense. The more people who are vaccinated, the less opportunity the virus has to replicate and spread. At some point, the community—or "herd" —is protected, even people who aren't immune or cannot be vaccinated, such as newborn babies, says the CDC.
What percentage of the population is needed for herd immunity?
Depending on how contagious an infection is, the Johns Hopkins Bloomberg School of Public Health says 50% to 90% of a population must be immune before herd immunity is achieved. Herd immunity against measles, for example, requires about 95% of the population to be inoculated; for protection from polio, it's about 80%, according to the World Health Organization (WHO).
Since COVID-19 is caused by a new virus, the percentage of the population that must be vaccinated to achieve herd immunity is unknown. WHO says a "substantial" proportion of the population would need to get their shots. Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases and a Biden administration adviser, has been revising estimates upwards from 60% to 70% in the pandemic's early days to close to 90% more recently, the New York Times reports.
Is herd immunity possible with COVID?
In theory, widespread COVID-19 vaccination should tamp down disease transmission in the community. But there are a number of wild cards.
For one, the emergence of new variants of SAR-CoV-2 (the virus that causes COVID-19) means that the infection is becoming more transmissible. Herd immunity also hinges on how quickly the nation gets shots into arms and how long that immunity lasts, the Times points out.
That's why, for the time being, the usual mitigation measures—washing hands, wearing masks, watching your distance from others—remain as crucial as ever. This is not the time to let down your guard.
The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.
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