Medical Examiner

Why Antibody Tests Can’t Tell You if You Should Get a Booster Shot

They cannot really measure how strong your COVID immunity is.

A phlebotomist wearing PPE draws blood for an antibody test in Santa Fe Springs, California, on April 21
Frederic J. Brown/Getty Images

Whether the vaccinated should get booster shots is already a scientifically, ethically, and strategically vexing question. Somehow, the federal government has made it more baffling by communicating its policy with the clarity of a terms of service agreement: In August, the Biden administration announced boosters would become available for all Americans who received the mRNA vaccines, starting in September. But then the Food and Drug Administration reviewed the data and scaled back the recommendation to only include the medically vulnerable, those over 65 years of age, and front-line workers, at least for Pfizer recipients, and we’re still waiting on recommendations for other vaccines. The mixed messages have meant that plenty of vaccinated people who aren’t eligible for the booster think getting one anyway might be worthwhile, and have been tempted to take matters into their own hands.

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To determine the precise right time to get a booster, the curious are turning to antibody tests. Antibodies, after all, are the immunological foot soldiers that ambush troublesome pathogenic invaders before they wreak too much havoc in the body. It stands to reason that if your coronavirus antibody test numbers are low, you’d probably benefit from a booster to get them up.

The problem is that antibody tests may not be a reliable way to track immunity in any detail—despite being the hottest new health metric in the Hamptons. Unlike the PCR-powered nasal swabs that reveal if you’re currently infected with SARS-CoV-2, the virus that causes COVID, antibody tests tell you if you have some immunity to SARS-CoV-2 from a prior infection or vaccination. The right antibody test is pretty good at answering this as an either-or question. It’s when you want more quantitative information that things get murkier. There are some readings that would be clear: Say we’re working with a scale of 0 to 2,500. If your one-month test reading gives you a count of 2,500 but six months later the reading has dropped to 0—an unlikely scenario for non-immunocompromised adults—you might be concerned. If your one-month test reads 2,500 and then six months later your reading remains steady at 2,500, you’re probably still well protected. But if your one-month test is 2,500 and then six months later your reading is, say, 800—well, what does that mean?

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I posed this question to Dr. Dorry Segev, a transplant surgeon and director of the Epidemiology Research Group in Organ Transplantation at Johns Hopkins who’s led multiple studies on COVID vaccination in immunocompromised patients. He put it this way: If your antibody score drops from 2,500 to 800, “you’re probably less protected in the immediate sense than you were six months ago, but six months ago, were you superhero-level protected, and now you’re only normal-person-level protected?” Is normal-person-level protected enough? “In the middle range, it’s hard to advise people what to do,” Segev said. So even though recent data show that, on average, lower antibody neutralization levels mean less protection from symptomatic infection, it can be difficult to translate that trend in the data to immune protection in each individual.

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There are a few reasons for this. The first is each person generates a unique set of antibodies in response to natural infection or vaccination. Some people’s antibodies will deftly subdue the virus, while those of others may have to wrestle with it for a bit. Scientists haven’t fully characterized this person-to-person variation, so in principle two people could have equal protection if one person has relatively low levels of powerful antibodies, and the other has high levels of less powerful but still adequate antibodies. It’s not clear exactly how antibody test numbers reflect this variation.

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A second reason is antibody levels spike near infection (or vaccination) and then naturally diminish over time. But that doesn’t mean antibody protection is gone. Your immune system has a “just-in-time” manufacturing strategy when it comes to antibodies. After infection or vaccination, most leftover antibodies are gradually broken down, but if the pathogen returns, cells called memory B cells can ramp up production again. Antibody tests only measure the current antibody inventory, so to speak, not the manufacturing capacity. With SARS-CoV-2, scientists don’t quite know yet how the two relate, so it’s difficult to interpret exactly what an antibody test means for immune protection.

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Finally, antibodies aren’t the immune system’s only line of defense. Our innate immune system is a vanguard battalion that swiftly surrounds any invading pathogen—but the strength of innate immune systems varies from person to person. A vaccine will do more than produce antibodies; it will also train T cells to fight the virus. These backup defenses may not be mobilized rapidly enough to prevent infection, but they could prevent the virus from inflicting serious damage. And although it’s possible to measure the levels of these cells in the blood too, that doesn’t paint the full picture either, said Dr. Donna Farber, a Columbia University immunologist who studies T cells. Many of these cells circulate deep in our tissues, beyond the reach of tests. “We can’t ever be completely sure just how much immune protection that you have to this virus,” she said.

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There’s no harm in checking in on your antibody levels, but they’re probably not going to help you decide whether a booster is needed. And more information isn’t always going to make you feel better, either. An uninterpretable number spit out by a random antibody assay could leave you with a false sense of vulnerability or a false sense of confidence, or just amplify the uncertainty you’re trying to tamp down on—none of which is particularly useful in a pandemic.

If wading through this medical morass has you exhausted, it’s because it shouldn’t really be falling on you to sort this all out. The public health authorities are failing, over and over, to communicate and justify their booster policy. Everybody is fretting about waning immunity and a drop in vaccine efficacy, but the evidence for either is far more ambiguous than the Biden administration would lead you to believe. Our antibody levels naturally wane over time, but it’s hard to determine whether decreased vaccine efficacy against infection is due to plummeting immunity, variants such as delta, or changes in our behavior. As Farber told me, “Everybody’s very worried. This virus is not going away in the way that we thought it would be. It’s just relentless. And so this is a way of trying to take some of that worry away.”

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Science aside, if you’re concerned about the ethics of getting a booster, it’s true that America has an immoral glut of vaccines during a global shortage. But the ethical calculus for individuals isn’t the same. There’s no infrastructure to reroute the unused shots in America’s pharmacies to the rest of the world, meaning a lot of vaccine ends up in the garbage. I can’t tell you if it’s worth it to get a booster if it’s not recommended for you—but I can tell you that if you’re looking to get one, you can simplify the decision by just taking antibody levels out of the equation.

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