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.

Researchers think that the R0 for COVID-19 is between 2 and 3. This means that one person can infect two to three other people. It also means 50% to 67% of the population would need to be resistant before herd immunity kicks in and the infection rates start to go down.

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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.

Early research shows that you can’t get the infection more than once. This is important for lasting herd immunity. But since this virus is so new, more research is needed.

With no vaccine to protect against COVID-19 yet, a large number of people would need to catch the virus, get sick, and recover before we can have herd immunity.

WebMD Medical Reference Reviewed by Arefa Cassoobhoy, MD, MPH on March 23, 2020

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.”

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