| You might remember antibody tests from the frenzy over them early in the pandemic, when the now-ubiquitous PCR screenings were scarce. Designed to detect prior coronavirus infections using a blood sample, antibody tests helped bridge the gap for people who thought they might have been infected. Initial enthusiasm waned over time, but now antibody tests are having a second life, albeit a questionable and possibly useless one, as a means of checking whether someone's Covid-19 vaccine has worked.
At the heart of the matter is this: Approved vaccines for Covid-19 are extremely effective, but even the best ones don't work in 100% of all cases. That has created doubt among consumers that makers and processors of antibody tests like Labcorp, Quest and Roche are seeking to capitalize on.  A health-care worker takes a drop of blood for a Covid-19 antibody test. Photographer: Valerie Macon/AFP/Getty Images Testing giants Quest and Labcorp both describe their antibody tests as something that can be used in the context of vaccination, though their websites include disclaimers about whether the results are medically relevant. The Swiss drugmaker Roche, meanwhile, said a new type of screening it launched last year would play an important role in measuring people's responses to Covid shots. The rub is that there's not yet sufficient research to support the notion. And the Food and Drug Administration has signaled that these marketing strategies may be premature. Antibody test results "should not be used to evaluate a person's level of immunity or protection from Covid-19 at any time, and especially after the person received a Covid-19 vaccination," the FDA said in a statement last month. Scientists say they're worried. Someone might decide not to go back to work, for example, if they believe their vaccine hasn't provided adequate protection, or might prematurely drop all precautions if the results suggest the opposite. No one, they say, should make an important life decision based on misleading data.—Emma Court | 
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