How Helpful Is COVID-19 (Coronavirus) Antibody Testing?

If you think you were exposed to COVID-19 (coronavirus), being tested for antibodies to the new virus could bring you peace of mind — but there are no certainties.

Antibody testing is useful for public decision-making. When it comes to your own health, every case is different.

In the midst of a pandemic, everyone would like to know that they are safe. Things are very uncertain, and uncertainty is uncomfortable. It’s clear that people react differently to COVID-19. Some people don’t notice more than a sniffle, while others feel sick for a week or two. Some people need hospital care. Others — even if they didn’t require hospital care — continue to cope with exhaustion and fevers for weeks or months. Too many die.

YOU MIGHT ALSO LIKE: Our Cold and Flu Season section

If you think you were exposed but were never especially sick, you may long for an antibody test to tell you that you’re one of the lucky ones. A positive result is a sign that you were exposed to the virus and mounted an immune response.

Antibodies are proteins your immune system makes to fight against infection. They stay in your blood after an infection has ended, ready to bind to the virus and kill it or mark it for death to protect other immune cells. Testing for antibodies sees whether your immune system has responded against an infection.

If the coronavirus is like others in that family, exposure brings you immunity, although researchers don’t know for how long. Having antibodies suggests that you have less reason to fear getting sick, and, if you get sick, less reason to fear a more serious reaction to the coronavirus. A negative result might be a reason to continue to prevent exposure to the virus.

The problem: This logic may not necessarily play out as expected for you. An antibody test result can be misleading.

How antibody tests could mislead you

Here’s what you need to know to evaluate whether you should take a test or what conclusions you could draw from it.

You can see the Centers for Disease Control and Prevention guidelines for antibody testing here.

The test you take may not give you an accurate result. No test is perfect. But there are hundreds of antibody tests, and many of them are especially unreliable. Be wary of a rapid test — when you get the result right away.

Some people who’ve had symptoms that could have been COVID had negative antibody results, but that does not mean they did not have the virus.

Of 12 antibody tests studied by the COVID-19 Testing Project, one of the tests produced false positives more than 15 percent of the time. That means about one in seven people got the wrong information. Three other tests produced false positives in more than one in 10 people. After hearing complaints of a flood of bad tests, the Food and Drug Administration announced that government agencies would collaborate to evaluate the tests.

Why might you get a false positive?

One reason is that there are four other coronaviruses that circulate in people, causing about a quarter of all common colds. COVID has mutated into several variants, as well, just like the flu does. A test might mistakenly pick up antibodies you developed after last year’s cold.

Even the best test will have some false positives and false negatives.

An antibody test cannot tell you whether you are contagious. Antibodies take a while to develop and linger in your body after recovery, so they aren’t a clear indicator of an active infection. Researchers also don’t know whether antibodies mean you aren’t shedding the virus, sending it into the air when you talk or cough.

It’s possible that you could have some immunity, and scientists don’t understand what that means. The antibody tests report levels (or titers) of antibodies. It’s not clear how high of a level you need to avoid a second infection, or how long your protection would last.

Studies of antibodies to other coronaviruses found that they provided immunity for a few years. Then it would begin to wane. Viruses mutate over time. Based on this history, epidemiologist Marc Lipsitch, D.Phil, suggests that some people might expect to be immune to COVID for about a year after an infection before their protection begins to wear off.

How antibody tests are helpful

Antibody tests are useful for public health officials who make decisions that affect the health of groups of people. In what’s called serological surveys, researchers analyze the results for a population to try to project what might happen if an area opens its beaches or bars. Again, they don’t know — they’re making a judgment.

In areas with the most cases, many people produce positive antibody tests. In one report, New York announced that one in five residents of New York City had tested positive in the state’s sample of 15,000 people statewide. In Westchester, the mostly wealthy suburbs nearby, the number was 13.8 percent. But in rural areas of the state, the percentage testing positive for antibodies was 2 to 3 percent.

Even the test used by a state hard-hit by the virus had a big range for error. According to Ashish Jha, MD, MPH, dean of the Brown University School of public health, nearly one in two results could have been false positives.

Around the world, most of these studies have found that small numbers — 2 to 5 percent of people — produce positive test results.

Researchers hope the coronavirus will stop spreading when enough people are immune, the more people the better. Measles, mumps, polio, and chickenpox used to be common in the United States until vaccines helped establish what’s called herd immunity, when the disease can’t spread. It requires 70 to 90 percent of the population to be immune to establish herd immunity. In areas where the vaccination rate falls, the disease breaks out again.

This history explains why scientists say we need an effective vaccine before people can go back entirely to normal. Antibody tests can help with public decision-making. But you’d be unwise to make any personal decision based on the current tests and knowledge.

YOU MIGHT ALSO LIKE: Our Infectious Disease section