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Should You Get an Antibody Test?

Questions And Answers to Help You Decide

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Immunity is the hottest buzzword in the American language right now and the fundamental mystery to solve is determining how people develop immunity, the key to which will be testing for antibodies in the blood. Antibody tests, sometimes referred to as serology, will help us identify antibodies to inform contact tracing; determine the effectiveness of vaccines; and clarify who may be susceptible to re-infection, and at what point, and why.

The basics of immunity—mechanisms most of us haven’t thought about since high-school biology, if ever—have suddenly been politicized to the point that they seem much more confusing than they actually are. Here is a basic question and answer guide to help you learn more and decide if antibody testing is right for you.

What is serology?

The study of serum. It mainly involves adding sero to the beginning of words (seroprevalence, seropositivity, seroprotection). These words are going to be a part of our lives for the next year or two, and they may sound technical, but basically all you have to do is subtract sero to understand them – they mean “we’re talking about antibodies.”

What does it mean if I have antibodies?

If you have antibodies to any virus, it means you’ve been exposed to that virus (or a vaccine for it). Your body remembers that exposure and will recognize the virus if you get exposed again. But having antibodies doesn’t necessarily mean you’ll be able to fight off a second infection. For that, you need sufficient numbers of antibodies, and they need to be effective antibodies. We don’t yet know the degree to which people with coronavirus antibodies are protected from getting COVID-19 a second or third time.

Do antibodies kill the virus?

Antibodies are proteins that float around in your blood and, essentially, look for things that are not right. If a virus has invaded and hijacked your cells to make thousands of clone viruses, for example, that’s not right. But the abnormality can be tricky to identify, because viruses can hide within our own cells. When a new, unknown invader proves difficult to distinguish, the body sometimes gets desperate and calls out the entire immune-system military—and also the National Guard, and the Boy Scouts, and anyone who has a pitchfork or a torch. The process can be fatal. Antibodies help prevent this by detecting the virus, or virus-infected cells, and binding to their surfaces, signaling the immune system to destroy only them. This allows for a precise, targeted killing that doesn’t do too much harm to healthy organs. The next time we see the virus, essentially, we just send out the Navy SEALs.

Will everyone develop antibodies?

Most humans have antibodies to the four coronaviruses that cause common colds, and it’s expected that antibodies to the new coronavirus will reliably develop in most people who are exposed to it. The question is how long they will last, and how consistently they’ll be effective at preventing a second case of the disease (if they are effective, they’ll be considered “neutralizing antibodies”). Typically you start producing some antibodies shortly after getting a virus. One kind, known as IgG, has reliably shown up on antibody tests in the second week of a COVID-19 infection.

How long do antibodies last?

We don’t know, but other coronavirus antibodies tend to last a few years. After the SARS coronavirus outbreak, in 2001, one study found that only 9 percent of people had antibodies six years after getting sick. They take time to develop, and they form only after you’ve been exposed to the virus. They are your blood’s unconscious memory of past infections. With most viral outbreaks, at least some of us have some degree of prior exposure—and, therefore, protection. But none of our immune systems has that memory this time around.

Do I have to get sick from the coronavirus to get antibodies?

No. This is the principle behind vaccination, whereby we try to create a situation that exposes you to enough of a virus that you develop antibodies, but not enough that you get sick. But it’s not clear that every exposure to the coronavirus will lead to antibodies. Though the amount of the virus that people are exposed to does seem to affect how sick they get, we don’t yet know how much the virus needs to replicate inside you before you develop antibodies.

How reliably do antibodies fight off the coronavirus?

This is the central question to answer in the coming months. Usually antibodies work very reliably. This is known as immune enhancement, a phenomenon that may or may not prove relevant with this coronavirus; it is worth keeping in mind when people suggest that antibody tests are currently painting a clear picture of who is totally protected from the disease.

Can I use someone else’s antibodies?

There is a lot of hope that this could be a useful treatment for people who get sick from COVID-19, or for very high-risk people who get exposed to the virus. We inject people with antibodies to prevent diseases like tetanus, so the idea isn’t unprecedented. Those antibodies instantly help neutralize the toxins in your blood after you step on a rusty nail, so you don’t have to endure two weeks of severe muscle spasms and lockjaw while your body makes its own. The approach is being studied now for this coronavirus; Tom Hanks even donated some of his antibody-laden plasma to the cause. However, for lasting protection, you need to make your own antibodies.

Can a test say whether I have enough antibodies to be protected?

This is measured in a specific type of antibody test known as a titer, which doesn’t just determine whether you have antibodies but counts the number in your blood. So the key questions will be what level is adequate to confer immunity, and under what conditions do people develop that adequate level? How reliably can we assume that if you have any antibodies, you are immune? That would mean you’re not only seropositive but seroprotected.

Why are we putting so much emphasis on antibody testing if the results don’t necessarily mean I’m superhuman?

Right now, the antibody tests are being used to help map out where the coronavirus has spread, like tracking the footprints it has left. Combined with other types of research, this information will eventually help identify who is most susceptible to infection, and why. Even if we can’t tell individuals that they are totally protected, we could theoretically begin to allocate scarce resources away from a city where 50 percent of people have antibodies to one where only 5 percent of people do.

That means if we have a lot of positive antibody tests, we can stop social distancing?

One day, yes, hopefully. But we don’t know how many people with antibodies are truly protected from the coronavirus yet. Antibodies wane over time, and not everyone has the same, lasting response to disease or vaccination—as we’ve seen with diseases like measles and hepatitis B. We don’t know how reliably people who are infected by the coronavirus develop effective antibodies. Figuring that out requires longer-term studies of who gets sick twice, and what sort of antibody response is needed to prevent that.

What percentage of people would need to have antibodies—effective ones, in effective amounts—to completely reopen society?

That comes down to the concept of herd immunity. With a disease like measles, not everyone has complete immunity by way of vaccination (because people’s antibody response to the vaccine waned over time, or because they have refused vaccination in the first place). But except for occasional, local outbreaks, we still collectively have enough antibodies that the virus can’t take hold and cause a pandemic. In the same way, the annual flu season ends as we approach herd immunity to that year’s strain of influenza.

The percentage of people required to reach herd immunity varies based on the virus. For a more contagious virus, we need higher percentages of the population to be immune. Determining that percentage comes down to the “basic reproductive number,” or R0, which is the average number of people who will catch a disease from any given contagious person. There’s still disagreement about what that number is for the new coronavirus, but at the moment many experts estimate that it’s between two and three. In any case, one basic approximation of herd immunity is when you multiply the R0 by the proportion of the population that is not immune and the result is less than one.

Does all the mask-wearing and social distancing mean we’re going to take longer to get to herd immunity?

Those basic measures can and have helped to lower the R0. If we keep doing them, we could have a relatively low percentage of immune people and still open businesses back up, because the disease would effectively be less contagious. People would get COVID-19, but we wouldn’t be flirting with catastrophic exponential growth.

What’s the best estimate for the percentage of immune people across the United States right now?

The rate of positive antibody tests varies widely from place to place. And those tests are limited and we don’t know what they mean. In Chelsea, Massachusetts, a small, early study appeared to show a roughly 30 percent positive rate, while in Santa Clara County, California, the positive rate was about 3 percent. In either case, there’s no evidence that we are near herd immunity. And, again, these studies weren’t meant to measure immunity; they were only meant to measure exposure. Measuring immunity will mean seeing how many of the people with antibodies end up getting sick again.