60,000-plus people have been tested for COVID antibodies in Mass. But the state won’t say how many tested positive

Residents of Chelsea were tested for antibodies in Bellingham Square in April.Stan Grossfeld/ Globe Staff

Tens of thousands of people across Massachusetts have been tested for antibodies linked to COVID-19, offering what researchers say could be vital information on how far the disease has spread.

But the state, despite recently disclosing how many new tests are being performed, has made little else public, including how many of the 61,000 people who’ve received antibody tests have tested positive.

Even that answer, infectious disease experts say, begs more questions, from who received tests to where — none of which the state provides publicly while releasing other detailed data on the coronavirus’s spread.

While viral testing can detect whether someone is infected with COVID-19, antibody testing — also known as serology testing — can reveal if someone was infected in the past, even if they never showed symptoms or have recovered. That tool, doctors say, can provide a fuller picture of the virus’s reach through Massachusetts, and potentially crucial information about a person’s immunity.

The reliability of such tests, however, can be scattershot, and even Governor Charlie Baker has questioned the accuracy of antibody testing before the US Food and Drug Administration gives clear guidance on which tests are the “most efficacious.” A positive antibody test also does not indicate when the person was exposed, according to the state Department of Public Health.

“It’s definitely an evolving science,” Baker said this week.

Still, the state this month began incorporating data from antibody testing into its official count of cases, one of the crucial metrics Baker’s administration uses in gauging when to reopen sectors of the state’s economy.

Among the new infections the state now reports each day are “probable cases,” which can include people who had a positive antibody test and either had COVID symptoms or were likely exposed to a positive case, according to DPH.

The state does not specify how many of those positive tests there are each day, and, the “probable” tag can apply to other circumstances, too: people who didn’t have an antibody test but had COVID symptoms and were exposed to a positive case, as well as those who died and whose death certificates listed COVID-19 as a cause of death, even though they were not tested.

Since June 1, the state has disclosed the total number of people who’ve received antibody tests, which reached 61,085 as of Thursday. Roughly 15,400 have been reported since the start of the month.

But DPH has not released information on how many of those tests found antibodies, despite Globe requests this week for that information. Nor has the department described the populations of people that have received them, beyond a spokeswoman saying it includes people in “seroprevalence studies or if they had a compatible illness in the recent past” but didn’t receive a viral test.

Antibody tests can be offered at doctor’s offices, urgent care clinics, and private labs, where the typical cost can range from $50 to $120. Positive test results are reported to the state, the Globe has reported.

“Just knowing the numbers of tests being done is worthless information,” said Dr. David Hamer, an infectious disease expert at Boston Medical Center and professor at Boston University School of Public Health. “What are the populations being tested? Is it random sampling? And among those who are having random testing, it would be useful to have some degree of breakdown by age, sex, county — at least.”

That the state now has data on more than 60,000 tests is a large sample, said Dr. Michael Mina, an assistant professor of epidemiology at Harvard University’s T. H. Chan School of Public Health. But without knowing important underlying information, the results “can be very, very misleading.”

“As we’ve seen with a number of different attempts at this so far, releasing the data can go awry,” Mina said. "We want to make sure that we’re not selecting antibody tests that come from populations that are at potentially higher risk or lower risk, and then having people extrapolate that onto the whole population.

“What we need to do is develop very representative samples,” he said, “and how you do that is an art form and a science in itself.”

Mina said he and colleagues are setting up a program in association with Harvard University aimed at recruiting 5,000 people for longitudinal sampling, in which researchers would follow them over time. Other Boston-area researchers have launched or were awaiting approval for their own studies, while researchers at the Ragon Institute of Massachusetts General Hospital, MIT, and Harvard built their own test.

“In peace time . . . serological sampling is actually a very powerful tool because it gives you a lot of useful information for public health decision-making,” Mina said.

Smaller-scale studies have so far provided some clarity, even if results vary widely between communities. Researchers at Massachusetts General Hospital found in April that nearly a third of 200 Chelsea residents who gave a drop of blood on the street tested positive for antibodies.

MGH officials and the Boston Public Health Commission later tested 750 people from East Boston, Roslindale, and two sections of Dorchester over nearly two weeks in late April and early May, in what the scientists called representative sample of the city. That study found that roughly 10 percent of Boston residents have antibodies, indicating they fought off COVID-19.

Meanwhile, in Brookline, about 7 percent of nearly 690 residents tested were positive for antibodies, according to town officials. In that instance, the town initially had invited hundreds of randomly selected residents to take part in the program, but later opened it to all residents via social media.

“You can debate the quality of it,” Dr. John Iafrate, vice chairman of MGH’s pathology department and a member of the research team behind the Chelsea and Boston studies, said of having broader data. “But having some data would be infinitely better than having none, and guessing. You’re feeling the same frustrations that I’m feeling in academic settings: The absolute urgent need to gather data to make rational public health decisions.”

The role antibody testing will play in the state’s long-term response remains unclear. The market has been flooded with different tests, and whether the presence of antibodies confers immunity to the virus and if so, for how long, remain open questions.

“We need to know desperately whether people are protected by the presence of antibodies,” said Iafrate.

Baker said state officials are “hoping” to include antibody testing as part of their long-term surveillance strategy, including ramping up the state lab’s ability to conduct as many as 1,600 antibody tests each day. But the governor has also pointed to cautious advice from a group of outside advisors he convened, including former FDA commissioner Scott Gottlieb, Eric Lander of the Broad Institute, MGH leaders, and others.

“If you start getting into a grander scale use of this,” Baker said of serology testing, “I think we would like to get a little more positive feedback from the advisory board.”