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.,” Dr. 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