What Does a Coronavirus Antibody Test Actually Tell You?

“The idea that [having the antibodies is] a ticket or a passport or that you have a shield—that’s what makes me nervous,” Dr. Currier says, because that means those people will be less likely to follow precautions to reduce their risk for infection. As always, the best place to get specific recommendations for your unique case is your own doctor.

They can’t tell you anything about your personal level of risk for reinfection.

Although some experts say it’s likely you have some limited protection if you’ve recovered from the virus, that doesn’t mean you should walk around like you’re totally safe.

That’s partly because the actual potential for immunity isn’t conclusive yet. But it’s also because there’s the possibility that different people will develop different levels of protection based on a bunch of different factors that haven’t been teased out yet. For instance, results from a recent preprint looking at data from 175 recovered patients in China showed that people who were older had significantly higher concentrations of IgG antibodies in their plasma. Conversely, a few people in the study had so few antibodies that the test used in the study couldn’t pick them up. Overall, about 30% of people in the study developed very low levels of antibodies. What any of this actually means for our immunity remains to be seen.

They can’t tell you whether or not you’re still contagious.

In addition to the assumptions about immunity, there’s also the built-in assumption that having IgG antibodies means you’re no longer shedding the virus and, therefore, that you’re no longer contagious. Although that may be true for some illnesses, Dr. Kadkhoda says the timing of this IgG response—when patients may still be having symptoms—suggests that isn’t always the case for the new coronavirus. Instead, he says, we should keep using the CDC-recommended procedure involving nasopharyngeal swabs that look for viral RNA to determine whether or not someone is still contagious after an infection.

They can give us a better idea of how widespread the outbreak was and is.

One thing antibody tests are actually useful for is that, with enough accuracy, they can give us a better picture of how far the outbreak has spread.

We know that the virus has a long incubation period, meaning it may be up to 14 days before someone who has the infection actually has symptoms. But even if someone doesn’t feel sick or even if they never develop symptoms, they can still have the virus and spread it to other people. However, someone who had no symptoms or only mild symptoms might not ever get a COVID-19 test themselves or ever realize they had the infection, and therefore our estimates of the prevalence of the virus would have missed them.

So, using antibody tests, we can see who has actually been exposed to the virus, regardless of how severe their illness was or whether they ever got a COVID-19 test during their infection.

They can’t tell us whether or not it’s time to reopen a city.

Early on, politicians placed a lot of importance on getting early access to antibody tests. But they are definitely not the singular key to “reopening” society and safely sending people back to work, Dr. Kadkhoda says. Now the strategy has evolved.

In New York City, for instance, getting enough antibody tests for the population was just the first step in determining how widespread the outbreak was. And now that the city is looking to gradually reopen, Governor Andrew Cuomo set up criteria that each county must meet, including things like getting contact tracing up and running, making sure hospitals have enough overflow capacity, consistently seeing a low rate of new infections, and making sure health care workers have a large enough supply of PPE on hand.

The only way to know for sure if having COVID-19 once gives you protection is long-term research, including vaccine trials.

Ultimately, the only way to really know whether having antibodies to the new coronavirus in your system offers any immunity is to look at long-term studies. That includes epidemiological studies of real-world patients who have recovered to see whether or not, months later, they can be reinfected. For studies like these, data from other countries that dealt with the pandemic before the U.S. will be extremely useful. And, Dr. Kadkhoda says, that also includes long-term vaccine trials, which are currently underway, SELF explained previously.

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