In a retrospective study of patients tested for COVID-19, researchers at the University of Chicago (UChicago) Medicine found an association between vitamin D deficiency and the likelihood of becoming infected with the coronavirus.

The researchers looked at 489 patients at UChicago Medicine whose vitamin D level had been measured within a year before being tested for COVID-19, reports Xinhua.

Patients who had vitamin D deficiency, defined as less than 20 nanograms per milliliter of blood, that was not treated were almost twice as likely to test positive for COVID-19 compared to patients who had sufficient levels of the vitamin.

“Vitamin D is important to the function of the immune system and vitamin D supplements have previously been shown to lower the risk of viral respiratory tract infections,” said David Meltzer, Chief of Hospital Medicine at UChicago Medicine and lead author of the study. “Our statistical analysis suggests this may be true for the COVID-19 infection.”

Half of Americans are thought to be deficient in vitamin D, with much higher rates seen in African Americans, Hispanics and individuals living in areas like Chicago where it is difficult to get enough sun exposure in winter.

“Understanding whether treating vitamin D deficiency changes COVID-19 risk could be of great importance locally, nationally and globally,” said Meltzer. “Vitamin D is inexpensive, generally very safe to take, and can be widely scaled.”

The researchers are planning further clinical trials. They emphasize the importance of experimental studies to determine whether vitamin D supplementation can reduce the risk, and potentially severity, of COVID-19, as well as the need for studies of what strategies for vitamin D supplementation may be most appropriate in specific populations.

The study was posted on UChicago’s website on Tuesday.