First responders loaded a patient into an ambulance from a nursing home where multiple people have contracted COID-19 in Chelsea, which has the highest concentration of COVID-19 infections in the state. Scott Eisen/Getty

Nearly one third of 200 Chelsea residents who gave a drop of blood to researchers on the street this week tested positive for antibodies linked to COVID-19, a startling indication of how widespread infections have been in the densely populated city.

Sixty-four residents who had a finger pricked in Bellingham Square on Tuesday and Wednesday had antibodies that the immune system makes to fight off the coronavirus, according to Massachusetts General Hospital physicians who ran the pilot study.

The 200 participants generally appeared healthy, but about half told the doctors they had had at least one symptom of COVID-19 in the past four weeks.

Chelsea’s spike in coronavirus cases challenges hospitals and state

Public health experts already knew Chelsea had the state’s highest rate of confirmed COVID-19 cases and that the actual rate was probably higher. At least 39 residents have died from the virus, and 712 had tested positive as of Tuesday, a rate of about 1,900 cases per 100,000 residents, or almost 2 percent.

But the Mass. General researchers ― who excluded anyone who had tested positive for the virus in the standard nasal swab test ― found that 32 percent of participants have had COVID-19, and many didn’t know it.

“I think it’s both good news and bad news,” said Dr. John Iafrate, vice chairman of MGH’s pathology department and the study’s principal investigator. “The bad news is that there’s a raging epidemic in Chelsea, and many people walking on the street don’t know that they’re carrying the virus and that they may be exposing uninfected individuals in their families.”

Opinion | Chelsea, city of the working Latino immigrant, emerges as a COVID-19 hotspot

“On the good-news side, it suggests that Chelsea has made its way through a good part of the epidemic,” he said. “They’re probably further along than other towns.”

Scientists suspect that people who recover from COVID-19 may be at least temporarily immune from catching it again. Several biotechs and academic laboratories, in fact, are seeking blood donations from people who have recovered, in the hopes that their antibodies can help create a treatment or vaccine.

Chelsea’s city manager, Thomas Ambrosino, said he learned the results of the pilot study Thursday in a conference call with the researchers. He was dismayed, but not shocked.

Chelsea city manager sounds urgent alarm, calls for residents to stay home 24 hours a day

“We’ve long thought that the reported numbers are vastly under-counting what the actual infection is,” said Ambrosino, who has called his city the epicenter of the crisis in Massachusetts. “Those reported numbers are based on positive COVID-19 tests, and we’re all aware that a very, very small percentage of people in Chelsea and everywhere are getting COVID-19 tests.”

“Still,” he added, “it’s kind of sobering that 30 percent of a random group of 200 people that are showing no symptoms are, in fact, infected. It’s all the more reason for everyone to be practicing physical distancing.”

Indeed, one of the doctors who tested volunteers in Bellingham Square said it’s possible that some of the people who had the antibodies are still contagious.

“Just because you have the antibodies doesn’t mean you’ve cleared the virus,” said Dr. Vivek Naranbhai, a clinical fellow in hematology and oncology.

Researchers said the test results, which had yet to be shared with state officials late Friday, couldn’t necessarily be extrapolated for the city’s roughly 40,000 residents. Still, the findings provided a valuable snapshot of a community that medical experts say is especially vulnerable to COVID-19.

Chelsea covers only about two square miles, across the Mystic River from Boston. For generations, it has attracted new immigrants, and about 65 percent of its residents are Latino. Many live in three-decker houses, Ambrosino said, where it’s hard for people to isolate themselves. Many work in the hospitality industry and health-related fields, where exposure to the virus is greater. And a lot of them must go to work during the pandemic.

What it’s like to be a Chelsea cop in the age of coronavirus

To get Chelsea residents to participate in the study ― which included a questionnaire that was available in English, Spanish, and Portuguese — investigators allowed them to remain anonymous. But that meant none of the participants received the results of the blood tests.

The doctors used a diagnostic device made by BioMedomics, of Morrisville, N.C., to analyze drops of blood. It resembled an over-the-counter pregnancy test and generated results on the street in about 10 minutes. Although the test hasn’t won the approval of the Food and Drug Administration, Iafrate, the principal investigator, said Mass. General determined it’s reliable.

Within days, the physicians said, they hope to set up a medical tent outside the Mass. General Chelsea Healthcare Center to perform more antibodies tests with the device. The site will be located near a tent set up weeks ago to run standard PCR, or polymerase chain reaction, tests for people with active coronavirus symptoms. The latter uses nasal swabs to detect whether the virus is present at the time; the antibodies blood test reveals whether someone was infected in the past.

In addition to the new testing site, the researchers want to expand the study to other Massachusetts cities and towns. For all of those studies, Mass. General doctors plan to obtain the identities of participants so physicians can provide the results. But first the researchers need to come up with guidelines for what participants should do if they test positive for antibodies.

Dr. Dean Xerras, medical director of the Mass. General Chelsea Healthcare Center and a co-investigator in the study, said it illustrates why it’s essential for Chelsea and other communities to perform more tests, regardless of whether they detect antibodies or the virus itself.

“Knowing how many people are infected is critical,” said Xerras, a longtime member of the city’s board of health. “We need to get them isolated. We need to get masks delivered to the city. We need to launch more safe isolation sites. We need to be able to identify cases and then give people the things they need to prevent perpetuation of the spread.”

Chelsea and Revere officials, with help from the state Department of Public Health and the Massachusetts Emergency Management Agency, secured almost 150 rooms at a Quality Inn in Revere this week for residents who are recovering from COVID-19 and unable to isolate themselves at home, according to Ambrosino. So far, only a handful of those rooms are occupied.

Governor Charlie Baker on Thursday staunchly defended the state’s handling of the escalating outbreak in Chelsea, including asserting, without offering details, that city leaders have turned down help from his administration.