Understanding 'Herd Immunity' and Its Role in Ending the COVID-19 Pandemic

Here's what public health experts have to say about the possibility of achieving herd immunity in America.

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 a vast number of folks to fall ill with 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 the CDC 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 vaccinated people, 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.

Why Is Herd Immunity Important?

For many diseases, a certain percentage of the population must be capable of contracting the disease in order for it to continue to spread. However, once the proportion of the population that is immune to the disease exceeds the one capable of contracting it, the spread of the disease will slow down.

The percentage of a community that needs to be immune to achieve herd immunity varies from disease to disease. The more contagious an infection is, the greater the proportion of the population that needs to be immune to the disease to stop its spread. 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 virus we had no experience with, the percentage of the population that must be vaccinated to achieve herd immunity is unknown. Early in the pandemic, many scientists had thought that once people started being immunized en masse, herd immunity would permit society to return to normal. In fact, in 2020, Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases and a Biden administration adviser, estimated that 60% to 70% of the population would need to be immunized to reach herd immunity. However, as the pandemic evolved, Dr. Fauci's belief in achieving classical herd immunity faded.

Is Herd Immunity Possible?

The key to herd immunity is that there are too few susceptible hosts around to maintain transmission even if a person becomes infected. This is because those who have been vaccinated or have already had the infection cannot contract and spread the virus.

But there are a number of problems caused by the complexities of the virus.

For one, the emergence of new variants of SAR-CoV-2 (the virus that causes COVID-19) means that sterilizing immunity provided by infection or vaccination against diseases like measles is not possible. Sterilizing immunity means an individual can no longer be infected or infect others.

Vaccination, which helps us develop protective antibodies against future infection, is another pathway to herd immunity. However, while vaccines offer strong protection against severe illness, they are not 100% effective at preventing infection due to waning antibodies and emerging variants. Some fully vaccinated people will still get COVID-19 and can, therefore, transmit it to others. Herd immunity is only relevant if we have a transmission-blocking vaccine. If we don't, then the only way to get herd immunity in the population is to give everyone the vaccine.

As long as the virus spreads, it mutates—helping the virus survive and giving rise to new variants. Those mutants—such as Omicron—can become better at evading the protection people have from vaccines or an earlier infection.

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.