A member of myCovidMD takes a COVID-19 antibodies blood test at the Homeboy Industries parking lot in Los Angeles Wednesday, May 20, 2020. Credit: Damian Dovarganes | AP

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In early May, health care providers in Maine started offering a second kind of coronavirus test that could detect past infections, though they are not being recommended by the state’s top health official for most people due to questions over what the results mean.

These antibody tests are used to detect whether a person has had the coronavirus by detecting whether signs of it are in their blood. They differ from viral tests using a nasal swab or saliva samples to detect current cases and are expected to be a key part of the future response to the virus.

[Our COVID-19 tracker contains the most recent information on Maine cases by county]

However, it is unclear what the test results mean. It is unknown whether people who recover from the virus can get it again, the U.S. Centers for Disease Control and Prevention said. A recent study from The Netherlands found that immunity to the virus could only last six months.

That has led Maine CDC Director Nirav Shah to not recommend antibody testing for now. His agency, however, defers to health care providers to make the decision. As of last Wednesday, about 85 percent of all coronavirus tests conducted in Maine were viral tests, but 15 percent, or 2,884 tests, have been antibody tests.

“We still don’t have a good handle on whether the presence of antibodies confers protection for a couple of months, a couple of years or anything of that nature,” Shah said in a Saturday interview. “So my recommendation for anybody is still not to pursue antibody testing unless you’re part of a clinical trial or an organized study.”

Anyone who tests positive on an antibody test is logged as a probable case in the daily case counts that the Maine CDC reports, though they also have to have at least two symptoms associated with COVID-19, the disease caused by the coronavirus.

So, as their symptoms resolve, they are moved from probable cases over to recovered cases. As of last week, 125 of 187 probable cases have been moved to the recovered category, according to Maine CDC spokesperson Robert Long. If people who get positive antibody tests have no symptoms, they are not counted as cases, Shah said.

Antibody tests have been on the market for months now, but in May, the U.S. Food and Drug Administration started cracking down on fraudulent or ineffective tests after allowing dozens of manufacturers to sell tests that the agency had not approved.

Antibody tests manufactured by large health care companies like Abbott and Roche are relatively accurate, Shah said. But if someone gets a positive result on an antibody test, it does not necessarily mean that they are immune to the coronavirus. A positive test also does not indicate how many antibodies need to be present in order to be immune to COVID-19 and for how long antibodies will offer protection.

Inaccurate antibody tests can also yield false positives, meaning the test might be detecting antibodies for viruses similar to the new coronavirus. Tests can also generate false negatives, which means the test fails to detect antibodies when there are some present.

The U.S. CDC does not recommend using antibody testing results to gauge reopening of businesses or easing restrictions on social distancing. Antibody testing might play a bigger role in Maine’s testing strategy in the future, because it’s a rapidly developing area of research. Shah said last week that the state is “looking at it very closely.”

While antibody tests interpretations remain under scrutiny for accuracy, there is a third category of tests, called antigen tests, emerging. These tests detect the presence of the viral protein in blood or another biological sample, like a saliva or nasal swab. Antigen tests already exist for many viral infections like influenza, tuberculosis and HIV. But so far, the FDA has approved only one antigen test.

Not much is known about antigen testing, but a main advantage is rapid results. An antigen test can provide results in minutes. Antigen tests may not detect all active infections, as they are not as precise as standard tests, which take longer but are more accurate, according to the FDA.

“I suspect that there will be widespread use of it before long,” Shah said.

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