Coronavirus Immunity and Reinfection

With no treatment or vaccine yet for COVID-19, there's been talk of using antibody tests to see when it might be safe to relax social-distancing guidelines. These blood tests can show who's been exposed to the virus and who hasn't. That's important because many people who have COVID-19 have mild or no symptoms.

The hope is that people who've been exposed to the new coronavirus will have immunity to it. When you have immunity, your body can recognize and fight off the virus that causes COVID-19. But it's possible that people who've had COVID-19 can get sick again -- and maybe infect other people.

We don't have enough information yet to know which is the case. But national health organizations are doing research to try to find some answers.

How Do We Become Immune?

When germs enter your body, your immune system springs into action. Here's how it works:

  • Bacteria and viruses like the one that causes COVID-19 have proteins called antigens on their surfaces. Each type of germ has its own unique antigen.
  • White blood cells of your immune system make proteins called antibodies to fight the antigen. Antibodies attach to antigens the way a key fits into a lock, and they destroy the invading germ.
  • Once you've been exposed to a virus, your body makes memory cells. If you're exposed to that same virus again, these cells recognize it. They tell your immune system to make antibodies against it.

Vaccines work in much the same way. They expose your body to an antigen that trains your immune system to fight that germ in the future. Because vaccines contain weakened or killed versions of viruses, you become immune without getting sick.

If You've Had COVID-19, Are You Immune?

Some countries want to issue "immunity passports" for people who have antibodies to the virus that causes COVID-19, called the SARS-CoV-2 virus. People with these "passports" would be allowed to go back to work and travel because they're supposedly immune to the virus.

But health experts don't yet know whether we really do become immune to COVID-19 after we're infected. And if we do become immune, we don't know how long that might last.

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Other types of coronaviruses appear to cause some immunity. Studies show that people are protected against the coronaviruses that cause the common cold for up to a year after an infection. And our bodies have antibodies against the SARS coronavirus for up to 4 years.

Most people who've recovered from COVID-19 do make antibodies against the virus. But so far, there's no evidence that this will protect them against the virus if they're exposed to it again.

In South Korea, more than 160 people tested positive again after they had recovered from COVID-19. In China, 5%-10% of people tested positive again after they'd recovered, according to news reports. It's not clear whether:

  • These people got infected again
  • The virus reactivated in their bodies after being quiet for a while, or
  • The test results were flawed

Could Herd Immunity Protect Us?

Herd immunity happens when a large part of the population -- the herd -- is immune to a virus. This can happen either because these people got vaccinated or had already been infected. Herd immunity makes it harder for a virus to spread. So even those who haven't been sick or vaccinated have some protection.

The more contagious a virus is, the more people need to be immune for herd immunity to kick in. The SARS-CoV-2 virus is so contagious that experts estimate about 70% of people in a community will need to be immune to have herd protection. That number might be hard to get to without a vaccine or a whole lot of people getting sick.

How Do we Test for Immunity?

Antibody tests, also called serology tests, measure antibodies to coronavirus in the blood. If you have antibodies, it means you've been exposed to the virus and your immune system has made antibodies against it. Antibody tests are different from the tests doctors use to check for the virus itself.

The FDA has approved four antibody tests for COVID-19. But more than 70 companies have antibody tests on the market. It's not clear how well these unapproved tests work.

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Because COVID-19 is so new, there hasn't been much time for scientists to check the accuracy of antibody tests. They could have false-positive results. That's when someone tests positive for antibodies but hasn't really developed them.

Testing for antibodies too soon after an illness can also cause false results. It takes 5-10 days after you get infected to develop antibodies against the SARS-CoV-2 virus.

Antibody tests could give people a false sense of security. They might go back to work and start to travel again when they could still catch or spread the virus. And because people can pass COVID-19 to others without showing symptoms, false positive results could lead to more outbreaks of the virus.

WebMD Medical Reference Reviewed by Arefa Cassoobhoy, MD, MPH on April 27, 2020

Sources

SOURCES:

Children's Hospital of Philadelphia: "Types of Immunity."

FDA: "Coronavirus (COVID-19) Update: Serological Tests," "Coronavirus (COVID-19) Update: Serological Test Validation and Education Efforts."

Harvard Medical School: "If you've been exposed to the coronavirus."

Immunologic Research: "T cell-mediated immune response to respiratory coronaviruses."

Johns Hopkins Bloomberg School of Public Health: "What Is Herd Immunity and How Can We Achieve It With COVID-19?"

Johns Hopkins Center for Health Security: "Serology-based tests for COVID-19."

KidsHealth.org: "Immune System."

MD Anderson Cancer Center: "7 things to know about COVID-19 antibody testing."

Microbiology Society: "Antibody-antigen complex."

New England Journal of Medicine: "Asymptomatic Transmission, the Achilles' Heel of Current Strategies to Control Covid-19."

University of California San Francisco: "The Promise and Uncertainties of Antibody Testing for Coronavirus."

Vaccines.gov: "Vaccines Protect Your Community."

World Health Organization: "'Immunity passports' in the context of COVID-19."

Canadian Medical Association Journal:" COVID-19: Recent updates on the coronavirus pandemic."

National Institutes of Health: "NIH begins study to quantify undetected cases of coronavirus infection."

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