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, 82,551 confirmed cases of COVID-19 had been reported as of Tuesday, 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