What Is Herd Immunity?

Herd immunity can help reduce the spread of some diseases

herd immunity
Verywell / Brianna Gilmartin

When new diseases strike, our bodies have no protection—or immunity—against them. As people get sick from and recover from these diseases, they develop immunity that can help keep them, and others, from getting sick again. Herd immunity—or community immunity—is based on the idea that as more people develop immunity to a disease, they help prevent the spread to others who are more vulnerable.

How Does Herd Immunity Work?

Herd immunity is something that happens when a large number of people in a community develop immunity—or the body’s own protection—against a contagious disease. This immunity can be developed naturally when the body makes antibodies after a viral infection that can help fight the infection better the next time. Herd immunity can also happen through vaccination.

Why Is It Important?

The idea of herd immunity is based on the thought that, as a community, we can protect our most vulnerable people. Newborns and people with compromised immune systems are one example. They can’t receive some vaccines, or they can’t develop immunity to diseases.

In the case of newborns, they receive vaccines in a scheduled series, and are susceptible to various diseases until they are fully immunized. For immunosuppressed individuals, their immune systems are weak, and they either cannot tolerate even the weakened virus contained in a vaccine, or they cannot mount immunity from the vaccine. Even healthy individuals may encounter vaccine failure—an estimated 2-10% of healthy people don't respond to vaccination.

Without natural immunity or immunity from vaccination, we can become very sick or even die if they are infected with a new disease for which effective treatments are unavailable. For example, in the case of the 2019 novel coronavirus (COVID-19), no one has immunity because it is a new virus and no vaccine is available. There are also no established treatments or cures, which is why the virus has caused a global pandemic. When enough people develop immunity against a disease, however, they can reduce the spread to the people that can’t develop immunity on their own or receive immunization in the form of vaccines. In other words, as more people develop immunity, fewer people are sick, and it becomes more difficult for the immunocompromised to come in contact with a sick person.

How Vaccines Help Herd Immunity

Vaccines can help create herd immunity by allowing our bodies to protect us from a disease without actually getting sick. Each specific vaccine trains the body’s immune system to detect and fight the targeted-disease. Vaccinating healthy people and reducing their ability to spread diseases protects people who can’t be vaccinated.

The eradication of smallpox is an example of herd immunity through vaccination. Vaccination for smallpox began in 1796 and became more widespread throughout the 1800s. Through vaccination, the last reported naturally-spread case of smallpox in the U.S. was in 1949, and the World Health Organization (WHO) declared the disease eradicated across the globe in 1980. In this situation, widespread vaccination decreased the number of individuals who spread the disease until the virus was no longer able to find suitable hosts.

How Effective Is Herd Immunity?

Herd immunity only works as well as the mentality of the herd. That is, it only works when large numbers of people are on board with the plan. According to the Association for Professions in Infection Control and Epidemiology (APIC), this is one of the main drawbacks of herd immunity, because it counts on the fact that people in the same community would share the same views on vaccination.

While natural immunity is possible, vaccination-driven immunity reduces illness most effectively.

One example of herd immunity failing due to a difference in beliefs is the case of measles in the mid-2010s. Cases spiked during that period, although measles had been declared eliminated in the U.S. in 2000. Pockets of infected individuals who had refused vaccination became infected and spread the disease to other vulnerable people. Therefore, someone who was was not vaccinated could become sick or carry a virus and pass it along to someone else who was unable to mount immunity or had also refused vaccination.

Another factor to consider is how well vaccines work, and how the immune system responds to those vaccines. For example, researchers have recently discovered that there is "waning immunity" for the measles-mumps-rubella (MMR) vaccine, particularly when it comes to mumps. Reports have found that even with a full vaccination and initial immunity, some people lose immunity to mumps over time. This can contribute to additional outbreaks, and has led to a recommendation for booster vaccinations when mumps outbreaks occur.

The efficacy of herd immunity depends on how many people participate, and it depends on how contagious the disease is. In the case of measles, which is highly contagious, research suggests that 93% to 95% of the population must be vaccinated to achieve herd immunity. In the U.S., average measles vaccination rates are above around 90% in most areas.

COVID-19: Can Herd Immunity Help?

There is some debate going on right now as to whether the spread of COVID-19 can be stopped by herd immunity. As world leaders debate strategies to contain and control this global pandemic, some have suggested herd immunity as an option. The United Kingdom briefly considered this idea, but models suggested it would require as much as 60% of the population to become infected with and then recover from coronavirus to provide herd immunity.

Because an effective vaccine is not currently available, developing herd immunity to COVID-19 would involve vast numbers of people getting infected. While many would recover—and create that herd immunity—many would die while sick. This has ignited an ethical debate about how many lives should be sacrificed for the benefit of others.

Many companies—ranging from Johnson & Johnson to biotech firms—are racing to create a COVID-19 vaccine. One company, Moderna, shipped experimental vaccines to the National Institute of Allergy and Infectious Diseases on February 24 and did a human trial in the U.S. on March 16. But a publicly available vaccine is still likely over a year away.

A Word From Verywell

Herd immunity—when available through vaccination—allows people to protect themselves, their families, and also the most vulnerable people in their community. Educate yourself on different types of vaccines, vaccine safety, myths and misconceptions, and suggested guidelines. Obtain your vaccination records from your primary care provider and make sure you are up-to-date.

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Article Sources
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