How Helpful Will COVID-19 (Coronavirus) Antibody Testing Be?

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.

Here’s the deal: 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, we’d all like to know that we’re safe. Things are very uncertain, and uncertainty is uncomfortable. It’s clear that people react differently to the new coronavirus that causes 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.

A vaccine for the virus began rolling out for Americans in December.

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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 made by immune system cells; your body makes them 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 from other immune cell. Testing for antibodies sees whether your immune system has responded against an infection. If the new coronavirus is like others in that family, exposure brings you immunity, although we 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 working from home and stay out of bars even if your area has opened up.

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.

The test you take may not give you an accurate result. One of my friends, who lives in a hot zone in Brooklyn, N.Y., was sick for a week with a high temperature, dry cough, and extreme fatigue. She lost her sense of smell and had waves of fatigue for a few weeks afterwards. She’d never experienced anything like it. Her doctor didn’t even suggest a test when she fell sick. Her doctor told her he’d been hearing about cases like hers all day long. However, her antibody test was negative.

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.

My friend got what she thinks is a false negative. If you get a false positive, you our told that you’ve had the virus. Yet, maybe you haven’t. 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 in mid-April 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. 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. We 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 we don’t understand what that means. The antibody tests report levels (or titers) of antibodies. We don’t know how high a level you need to stay clear of 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 the new coronavirus 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 because they are making decisions that affect the health of groups. In what’s called serological surveys, researchers analyze the results for a population to try to project what might happen if an area opened up 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. On May 2, 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.

However, 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.

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.

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