The key to rebounding from coronavirus may lie with antibody tests. But caveats abound.

As leaders strategize about reopening schools and businesses and plan for life on the other side of the so-called coronavirus curve, all eyes are on a type of testing that may help determine who has been infected with COVID-19 and whether they’re immune.

The test that may define this new frontier detects specific proteins in a person’s blood, known as antibodies, which develop to fight off infections such as COVID-19. The antibodies could help determine just how pervasive the disease is across the world, but also could potentially pinpoint whether an infected person who recovered has developed an immunity.

Though their accuracy remains in question, these antibody tests have become highly sought in the pathway out of the pandemic. Federal regulators have eased standards in an effort to speed production of the tests. Meanwhile, a Framingham company, believed to be the first in Massachusetts, is now distributing the test and says it has a dozen regional hospitals signed on to use it.

Still, disease specialists are wary of this emerging approach.

“We are hopeful that these tests will be the next good thing, but we don’t yet know what the unknown unknowns are,” said Dr. Harlan Krumholz,, a professor of medicine at Yale and director of the Yale New Haven Hospital Center for Outcomes Research and Evaluation.

Earlier this month, federal regulators approved the first antibody test, created by a North Carolina-based company. The US Food and Drug Administration’s newly relaxed rules allow companies to develop and distribute tests without FDA review, as long as the companies certify they have internally validated their tests and do not claim they are FDA-approved. And just last weekend, the Trump administration further paved the way for these tests, issuing guidance that they should be offered at no cost to those with health insurance.

Now dozens of companies are rushing out antibody blood tests. Employers, large and small, are eyeing the tests, and wondering how costly and available they’ll be. Still, Krumholz and other disease experts are uneasy about whether these tests will prove effective.

“It remains a bit of the Wild West out there,” Krumholz said. “The tests are coming from a lot of different places, many from out of the country.”

It doesn’t help that officials in London last week acknowledged that millions of tests they ordered from China, designed for residents to use at home by pricking their fingers for blood, failed to meet UK standards.

The many unknowns are keeping disease experts up at night. How much of the antibodies does a person need to be immune to this coronavirus? How long could that immunity last? Will the tests mistake the novel coronavirus for another virus in a person’s blood?

“We’re not 100 percent sure,” said Dr. Shira Doron, an infectious disease physician and hospital epidemiologist at Tufts Medical Center “If you are fully immune, how long would that last. Is it just that season like flu? Or is it less? Or is it more?”

On Monday, World Health Organization officials said an early study suggests that not all people who recover from the coronavirus have a detectable marker in their blood. Their statements raise questions over whether patients develop immunity after surviving COVID-19.

The companies creating the tests are also grappling with these concerns.

“It’s not a test design that concerns us. It’s more understanding how the infection progresses,” said Greg Frank, director for Infectious Disease Policy at the Biotechnology Innovation Organizationwhich represents more than 1,000 biotechnology companies and academic institutions across dozens of countries.

Antibody tests are different than the tests now used to diagnose COVID-19, which swab patients’ nostrils to check whether they’re infected with the virus. An antibody test looks for proteins in a patient’s blood as evidence they were exposed to the coronavirus and potentially developed an immunity.

Antibody tests aren’t new. Doctors use them to help diagnose a number of illnesses, including arthritis, HIV, and lupus, and to determine if someone has developed an immunity to viruses such as the one that causes chickenpox.

The difference with COVID-19 antibody tests, Frank said, is that companies are learning about this coronavirus in real time, as it continues to spread around the globe.

Antibody tests typically “are developed when we understand more about the bug we are trying to look at,” Frank said. “But here there is a sense of urgency.”

Framingham-based Boston Heart Diagnostics said the antibody test it launched last week will initially only be offered to hospitals because they have a pressing need to know which health care workers may have immunity, knowledge that could help with staffing decisions and to ease workers’ fears.

“We are going to distribute the test to hospitals and leave it to them to decide who gets the testing,” said president Pat Noland.

Noland said the company has already contracted with 12 hospitals, which he declined to name, and said Boston Heart can run up to 4,800 tests a day. He acknowledged the many unknowns surrounding COVID-19 antibody testing, and noted that the company suggests hospitals wait at least 14 days after a patient is infected before administering the test. This way, their body will have enough time to produce antibodies.

While the state’s health department has worked with companies to open large-scale, drive-through sites that offer nasal-swab testing for diagnosing COVID-19, there is apparently no similar plan for antibody tests. Dr. Larry Madoff, the health department’s medical director of the Bureau of Infectious Disease and Laboratory Sciences, said there are still too many questions around testing.

“The COVID-19 Command Center’s medical advisory group is reviewing blood-based antibody testing in Massachusetts,” he said in a statement. “Like most lab tests, these will be available to providers who can assess their usefulness for a given individual.”

Employers across the state, already assessing how to safely and gradually reopen, have expressed a keen interest in antibody testing. But Jon Hurst, president of the Retailers Association of Massachusetts, a 4,000-member trade association, said as tantalizing as the tests seem, he’s worried about their costs and availability, especially to smaller employers, many already financially devastated by the pandemic.

“Whether it’s this test, or masks, or other forms of testing, what might be available to a BJ’s [Wholesale Club] or Walmart, for a national company, may not be the same for small businesses,” Hurst said. “If these tests are in limited supply, are they going to the [big companies] instead of mom and pop stores?”

The emergence of a new, sought-after test also raises questions about who gets it, and when.

A directive issued Saturday by the Trump administration requires health insurers to offer antibody testing at no cost to patients with insurance, but is unclear about what costs might be borne by employers.

The state’s largest health insurer, Blue Cross Blue Shield of Massachusetts, is reviewing the directive. The company’s chief physician executive, Dr. Bruce Nash, said whether the government pays for it, or commercial insurers pay, ultimately premiums are affected.

“As a leading national payer, we will work with national and state leaders to put policies in place for the best interest of the community,” he said.

Blue Cross, with 3,800 employees, is also one of the state’s largest employers. Nash said they, too, have many questions about the antibody tests and aren’t rushing to reopen until more is known about their precision and the virus itself.

“For an epidemiologist it’s a powerful tool — it may tell us how prevalent this is in our community — that would be the primary use,” he said. “But how do we reopen society using a test? That’s where it becomes more dicey.”

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