Nearly 3,300 inmates tested positive for the coronavirus across four state prison systems, with nearly all of them showing no symptoms, according to a survey.

Reuters spoke with officials and reviewed documents from state prisons in Arkansas, North Carolina, Ohio, and Virginia and found that out of the 3,277 inmates tested, 96% were asymptomatic.

“It adds to the understanding that we have a severe undercount of cases in the U.S.,” said Dr. Leana Wen, an adjunct associate professor of emergency medicine at George Washington University. “The case count is likely much, much higher than we currently know because of the lack of testing and surveillance.”

The survey comes as other areas in the country increase testing and find that more people had the coronavirus but showed no symptoms.

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Earlier this month, a random sampling study showed that at least 6% of the population in Miami has coronavirus antibodies, indicating past infection.

"What we're finding out is something we've frankly known all along. There are a lot of asymptomatic cases out there where people are carrying the virus but are not experiencing any symptoms," Republican Miami Mayor Carlos Gimenez said in a press conference.

In California, a random sampling test conducted by Stanford University researchers in Santa Clara County concluded the coronavirus is 50-85 times more prevalent than previously thought, and the fatality rate is 50-85 times lower.

Other countries are finding similar results, including in France, where a study determined that 25% of students and staff at a high school possessed coronavirus antibodies, with nine hospitalizations but zero deaths.