An overwhelmingly majority of inmates from four states' prison systems that tested positive for COVID-19 have been asymptomatic, according to Reuters.

Between Arkansas, North Carolina, Ohio and Virginia, 3,277 inmates tested positive for the virus. Of that number, 96 percent of the inmates were asymptomatic, meaning they showed no symptoms associated with the disease.

Asymptomatic patients can transmit COVID-19 unknowingly, putting at-risk populations in danger of contracting a virus that could be fatal. This fact has alarmed health experts since the onset of the pandemic and forced state government officials to instate social distancing measures.

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Asymptomatic carriers pose a particular challenge in the country's state prison system where often, inmates come in close contact with one another.

“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 the wire service. “The case count is likely much, much higher than we currently know because of the lack of testing and surveillance.”

The U.S. has more incarcerated persons than any other country in the world – over 2.3 million were reported in 2017. Michigan, Tennessee and California have all started mass testing in their respective prisons.

The results in those state prisons were similar; Tennessee reported a majority of its positive cases didn't show symptoms, while Michigan reported that “a good number” of the 620 positive cases showed no symptoms of the virus.

Some states, like Florida, New York and Texas are only testing inmates who exhibit symptoms.

“Prison agencies are almost certainly vastly undercounting the number of COVID cases among incarcerated persons,” Michele Deitch, a corrections specialist and senior lecturer at the University of Texas, told Reuters.

“Just as the experts are telling us in our free-world communities, the only way to get ahead of this outbreak is through mass testing.”

Overall in the U.S., COVID-19 has infected over 925,500 people, killing more than 53,000, according to data compiled by Johns Hopkins University.