Dr. David Meltzer, the study's lead author, said vitamin D deficiency is a common concern in Chicago.
"The fact that Chicago has such a large African American population, Hispanic population, and is located in a northern area where people don't get much sun exposure for most of the year," Dr. Meltzer said. "I think those all mean that there are areas where vitamin D deficiency is more likely here."
The research team looked at nearly 500 patients and said that those who had an untreated vitamin D deficiency were almost twice as likely to test positive for COVID-19.
The researchers said it's important to talk to your doctor before starting to take a supplement, because too much vitamin D can be dangerous.