Nearly 1 in 5 Chicago residents who sent blood-spot samples to Northwestern University researchers tested positive for antibodies to the coronavirus that causes COVID-19, according to preliminary results of an ongoing study.

That 20% infection rate is higher than the scientists anticipated based on earlier research, said Dr. Elizabeth McNally, director of the Center for Genetic Medicine at the Northwestern University Feinberg School of Medicine. One study by other Northwestern researchers tested hospital workers from across the Chicago region and found antibodies in less than 5%.

The latest project, called Screening for Coronavirus Antibodies in Neighborhoods, or SCAN, is examining infection rates in five pairs of adjoining Chicago ZIP codes where rates of previously reported COVID-19 cases differed widely. Though the research is continuing, McNally said enough testing has been done to draw some initial conclusions.

“It’s telling us that exposure was higher than we thought, that it was higher than we thought throughout Chicago,” McNally said.

The researchers noted that the SCAN team is using a particularly sensitive test for COVID-19 antibodies and therefore is likely identifying more exposures than others have found. The Northwestern researchers who tested hospital workers, for example, used a different antibody test.

“Those commercial tests are missing maybe 25% of people … whereas ours don’t,” McNally said.

In the ongoing study, participants mail in a drop of dried blood to the researchers, who then test it for the COVID-19 antibodies. It’s an inexpensive option that doesn’t require visiting a medical facility.

The SCAN study, launched in late June,set out to test about 3,000 people in 10 Chicago ZIP codes, and so far more than 1,000 tests have been analyzed, McNally said.Researchers are finding some differences in exposure rates among the ZIP codes but said they need to gather more test results before reaching any conclusions.

In the city as a whole, 83,074 confirmed cases of COVID-19 had been reported as of Thursday, or about 3% of the city’s 2.7 million people. The true number of people infected, however, is certainly far larger.

A study published in the Journal of the American Medical Association that was based on antibody testing concluded that actual COVID-19 infection rates were at least 10 times higher than the official reported case count in most locations around the country.

Official counts are based on tests that detect SARS-CoV-2, the virus that causes COVID-19. That type of testing can’t identify people whose infections have cleared. In addition, many infected people never get tested because they had no symptoms, had less severe symptoms or couldn’t access a test.

The National Institutes of Health has stated that a positive antibody test result “indicates previous infection.”

The antibody tests detect disease-fighting proteins that the body develops specifically to attack SARS-CoV-2. Those antibodies tend to persist for months or perhaps years in people who had a prior infection, although it’s not known exactly how long.

Another study by researchers on the SCAN team looked at infection rates among workers at the university’s Feinberg School of Medicine, who live in many Chicago and suburban locales. That study, which included workers who do not interact directly with patients, also found rates of about 20%, which McNally said appears to support the idea that the percentage of people affected is similar throughout the Chicago region.

Using the same sensitive antibody test, the researchers also conducted an earlier study of 232 people, most of whom lived with a health care worker. Of those, 30 lived with someone who was previously diagnosed with COVID-19.

That study, which has already been peer reviewed and published, found that, of the 202 people who had no previous diagnosis of COVID-19 and did not live in a home with a previously identified case, 36% had antibodies to the disease. That higher rate was not unexpected, given that so many health care workers were involved in the study.

They also found the infection rate climbed dramatically when people in the study were living in a home with someone who had been diagnosed with COVID-19. Among the 30 who had no prior diagnosis but were living with someone who did, 80% had antibodies, according to that study.

In addition to completing the testing on the 10 Chicago ZIP codes, the SCAN study is now being expanded to other parts of Chicago and suburban Cook County, McNally said. Researchers are still looking for participants.

There also are further questions to explore as the study continues to follow and research the people it has already tested.

“The $64 million question here is really, ‘Why do some people get so sick when exposed to this?’” McNally said, noting that genetics, diet, sleep habits, stress and other illnesses could play a role. “That’s what we have to answer.”

Researchers plan to ask study participants who had the antibodies to be tested repeatedly in the coming months to see how long those antibodies last and how well the antibodies protect people from subsequent infection.

It is thought that antibodies likely offer protection against reinfection, but many questions remain unanswered. Those include what level of antibodies are necessary and how long they last in people’s bodies.

“The most concerning thing” discovered so far, McNally said, is the number of folks tested who turned out to have had a prior infection but were unaware of that fact.

“They could be spreading it and not know they have it,” she said. “It shows how clever the virus is. And yet the solution that helps a whole lot is really quite easy: It’s called wearing a mask.”