About 10 percent of Boston residents have the antibodies that indicate they fought off COVID-19, according to a small-scale study, suggesting that 90 percent of people in the city have yet to be exposed to the virus.
Researchers from Massachusetts General Hospital and the Boston Public Health Commission tested 750 people from East Boston, Roslindale, and two sections of Dorchester from April 27 to May 8, in what scientists described as a representative sample of the city. People could participate only if they had no symptoms of the coronavirus at the time.
Mayor Martin J. Walsh of Boston and the researchers said the findings suggest that social distancing, the wearing of face masks, and frequent hand-washing are curbing infection rates. But if having the antibodies confers immunity ― something that has yet to be confirmed ― the results also mean that most residents of Boston remain vulnerable to catching the disease.
As a result of the findings, Walsh said he had no plans to lift the public health emergency order he imposed on March 15 “next week or in the near future,” even as the state moves closer to reopening.
“I know that many people are feeling a bit worn down after living through two months of this,” he said. “But the fastest and the most sustainable way out of this situation is a healthy way. If we come back too soon, there will be a second surge.”
Although the results varied somewhat depending on where people lived, Walsh said, "What this study showed us is that 90 percent of Bostonians have not been impacted by COVID-19, as far as their physical health.”
The researchers sought to evaluate exposure to the virus that is blamed for 5,592 deaths in the state as of Friday. Participants were tested at drive-through sites in East Boston, Roslindale, and Dorchester.
About 38 percent of the participants were from Dorchester, nearly 37 percent from Roslindale, and 25 percent from East Boston. The median age was about 42.
Results varied by neighborhood. The highest percentage of people with antibodies was found in East Boston, at 13.3 percent; the highest percentage of people with an actual infection was 4.6 percent, in the 02125 zip code area of Dorchester.
Roughly 62 percent of the participants were women, and 38 percent men. About 62 percent were white, nearly 19 percent Black, and about 12 percent Hispanic.
Researchers found that 9.9 percent of the participants who gave a drop of blood had antibodies linked to COVID-19, indicating past exposure to the coronavirus but not an active infection. In comparison, when MGH scientists tested 200 people in Chelsea’s Bellingham Square in mid-April, nearly one third tested positive for antibodies.
In the Boston study, Mass. General researchers also took nasal swabs from participants to test for an active coronavirus infection. This yielded a troubling finding: 2.6 percent of volunteers tested positive for the illness even though they had to be asymptomatic to participate.
“There’s lots of people walking around who could be infectious, one in 40,” said Dr. Vivek Naranbhai, a Mass. General clinical fellow in hematology and oncology, who helped lead the research team. He called the results “sobering.”
More than 5,000 residents living in East Boston, Roslindale, and within the boundaries of zip codes 02121 and 02125 in Dorchester were invited to volunteer for the study. In the end, 750 received a finger prick for the antibodies test and gave a nasal swab for the test for an active infection.
Dr. John Iafrate, vice chairman of MGH’s pathology department and another leader of the research team, said there was no significant difference in antibodies results based on ethnicity or type of job. People were most likely to test positive if they thought they might have had COVID-19, reported a fever in the past month, or had experienced a change in taste or smell.
As in Chelsea, residents of the four neighborhoods were more likely to test positive for the antibodies if they lived in densely populated areas.
“The good news,” he said, “is the percentage of people who have been infected is lower than would have been projected two months ago. It looks like we flattened the curve. The bad news is we’re nowhere near herd immunity.”
That refers to the percentage of people in a community who need to be immune to a infectious disease ― either through a vaccine or antibodies manufactured by the body while fighting off an infection ― to make the illness unlikely to spread to others who lack immunity.
Some scientists believe that 50 to 70 percent of people need to be immune to COVID-19 to minimize the risk to people without immunity, Iafrate said.
There is no approved vaccine for COVID-19 although more than 100 vaccine candidates are in development.
Dr. Michael Mina, an epidemiologist with the Harvard T.H. Chan School of Public Health, said in a Facebook Live session on the epidemic Friday that the communities sampled might not be representative of Boston as a whole. He suspected that a citywide antibodies testing program would reveal a smaller rate of exposure to COVID-19, closer to 4 percent.
Iafrate acknowledged that the study was small but called it “an approximation” of the city.
In another study, Mass. General researchers on Thursday tested 224 Brookline municipal employees for COVID-19 antibodies as well as for current infections, according to Swannie Jett, the town’s health commissioner. They were all classified as essential workers in the police, fire, public health, and public works departments.
Sixteen of the 224 workers tested positive for the antibodies after giving a drop of blood, Jett said. Of those, nine showed signs that they might actually have the virus in a second test of the blood sample. He said he was directing those employees on Friday to isolate themselves at home for 10 days and recommending they undergo nasal swab tests to confirm they had the infection.
Martin Finucane and Felice J. Freyer of the Globe staff contributed to this report.
Jonathan Saltzman can be reached at email@example.com