What Is Herd Immunity?

With the rising number of cases of COVID-19 around the world, health officials continue to work to find the best way to protect the public from the disease. You may have heard health officials mention herd immunity as a possible way to contain the spread of COVID-19.

Here’s what you need to know about herd immunity and how it may help slow the spread of the new coronavirus.

Herd Immunity

Herd immunity, or community immunity, is when a large part of the population of an area is immune to a specific disease. If enough people are resistant to the cause of a disease, such as a virus or bacteria, it has nowhere to go.

While not every single individual may be immune, the group as a whole has protection. This is because there are fewer high-risk people overall. The infection rates drop, and the disease peters out.

Herd immunity protects at-risk populations. These include babies and those whose immune systems are weak and can’t get resistance on their own.

How Do You Achieve Herd Immunity?

There are two ways this can happen.

You can develop resistance naturally. When your body is exposed to a virus or bacteria, it makes antibodies to fight off the infection. When you recover, your body keeps these antibodies. Your body will defend against another infection. This is what stopped the Zika virus outbreak in Brazil. Two years after the outbreak began, 63% of the population had had exposure to the virus. Researchers think the community reached the right level for herd immunity.

Vaccines can also build resistance. They make your body think a virus or bacteria has infected it. You don’t get sick, but your immune system still makes protective antibodies. The next time your body meets that bacteria or virus, it’s ready to fight it off. This is what stopped polio in the United States.

When does a community reach herd immunity? It depends on the reproduction number, or R0. The R0 tells you the average number of people that a single person with the virus can infect if those people aren’t already immune. The higher the R0, the more people need to be resistant to reach herd immunity.

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In 2020, researchers estimated the R0 for COVID-19 to be between 2 and 3. This meant that one person could infect 2 to 3 other people. It also meant that 50% to 67% of the population would need to be resistant before herd immunity would kick in and infection rates started to go down.

New variants like Delta raised the R0 to between 5 and 7. Omicron has proven far more contagious, with an R0 estimated to be between 15 and 21 -- about 3 times higher than Delta.

What Are the Challenges to Developing Herd Immunity to COVID-19?

The main obstacle to herd immunity to COVID-19 right now is that the virus that causes the disease is “novel,” or new. That means that it hasn’t infected humans before and everyone is at risk of infection. There’s no existing immunity to build on.

Another potential barrier is that we don’t know how strong the immune protection is or how long it will last in people who’ve had COVID-19. Early research on monkeys showed that they made antibodies to the virus that protected them from a second infection a month later. If the coronavirus is like the flu, we can expect a few months of protection.

While there are now vaccines to protect against COVID-19, It may be months before enough people can receive them. It is hoped, that the vaccines will eventually help bring the spread under control. Researchers estimate that 75-80% of the population would need to be vaccinated before we can have herd immunity.

What’s The Best Path to Halt Community Spread?

Vaccination. There are three effective vaccines from Pfizer, Johnson & Johnson, and Moderna that work well, both to stop infection and to lessen serious illness if you do get infected. Get vaccinated and keep up with your boosters to keep your immunity up. It’s by far the best way to protect yourself and others from infection from COVID-19. And it appears to be the best path to help stop community spread and keep you from getting severe disease.

WebMD Medical Reference Reviewed by Melinda Ratini, DO, MS on February 03, 2022

Sources

SOURCES:

CDC: “Vaccines and Immunizations: Glossary,” “Frequently Asked Questions -- Coronavirus Disease 2019 Basics,” “Understanding How Vaccines Work,” “Polio Elimination in the United States.”

Science Media Centre: “Expert Comments About Herd Immunity.”

Association for Professionals in Infection Control and Epidemiology: “Herd Immunity.”

John Hopkins Medicine: “The Immune System.”

mBio: “High Zika Virus Seroprevalence in Salvador, Northeastern Brazil Limits the Potential for Further Outbreaks.”

Centre for the Mathematical Modelling of Infectious Diseases, London School of Hygiene & Tropical Medicine: “Critical Immunity Thresholds for Measles Elimination.”

Journal of Travel Medicine: “The reproductive number of COVID-19 is higher compared to SARS coronavirus.”

National Institutes of Health: “NIH clinical trial of investigational vaccine for COVID-19 begins.”

Imperial College London: “Impact of non-pharmaceutical interventions (NPIs) to reduce COVID-19 mortality and healthcare demand.”

Journal of Medical Virology: “Coronavirus infections and immune responses.”

Penn State Huck Institutes of the Life Sciences: “COVID-19 FAQ: Ask CIDD.”

American Lung Association: “From the Frontlines: Understanding Herd Immunity.”

Virginia Department of Health: “COVID-19 and Influenza Surveillance.”

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