96% of 3,277 inmates in state prison systems in Arkansas, North Carolina, Ohio and Virginia who tested positive for the coronavirus did not show symptoms, Reuters reports.

Why it matters: The findings suggest that asymptomatic patients could be contributing to the spread of the virus at a far higher level than suspected, especially in prisons, which have proven to be a breeding ground for COVID-19 in the U.S.

This is especially noteworthy because health officials have prioritized testing and surveillance for people that show symptoms. 4,693 total tests were conducted.

“It adds to the understanding that we have a severe undercount of cases in the U.S.,” Leana Wen, adjunct associate professor of emergency medicine at George Washington University, told Reuters.

The big picture: Two prisons — Marion Correctional Institute in Ohio and Chicago's Cook County jail — are among the largest known sources of coronavirus infections in the U.S.

At Marion, close to 95% of the 2,028 inmates who tested positive for the virus were asymptomatic, according to Reuters.

Go deeper: Coronavirus behind bars