Early in the coronavirus pandemic, public health experts singled out prisons and jails as severe outbreak risks, prompting calls to release inmates to mitigate its spread.

Those warnings proved prescient. Eight of the 10 largest clusters of the novel coronavirus in the United States are now in jails and correctional facilities, according to data compiled by the New York Times. And new peer-reviewed research, published in the journal Health Affairs, shows that those outbreaks have ripple effects that extend far beyond institutional walls.

As of mid-April, the study found, nearly 16 percent of coronavirus infections in Illinois could be linked to a single source: the Cook County Jail in Chicago. The authors say the data highlight how our punitive criminal justice system — with its particular reliance on arrest and incarceration — is a risk factor that makes everyone in the country more vulnerable to a pandemic.

The researchers obtained data from the Cook County Jail on the entrance, release and covid-19 status of thousands of inmates who were incarcerated in the facility in March. In contrast to prisons, which typically house long-term inmates serving out their punishment, most jail stays are brief: The majority of jail inmates are accused but not convicted individuals awaiting their day in court.

As a result, there is considerable “cycling” between jails and the communities around them. People are arrested, housed temporarily in cramped conditions highly conducive to the spread of infectious disease, and then either released back into the community or sent on to other detention centers to serve out their sentences. Staffers who make daily trips between jail facilities and their homes represent another possible vector of community spread.

The researchers examined the Zip code-level relationship between the presence of released Cook County Jail inmates and the rate of coronavirus infection. They controlled for a number of potentially confounding factors, including poverty, public transit use, population density and race.

Their main conclusion was “the cycling of 2,129 individuals through Cook County Jail in March was associated with 4,575 additional known community infections in Illinois as of April 19,” or nearly 16 percent of all cases across the state at the time.

This may be an undercount. Prison staffers accounted for more than 100 of Cook County Jail’s 600-plus coronavirus infections as of April 19, but the researchers did not have data on staffers’ Zip codes of residence, meaning the analysis is blind to potential community spread linked to jail employees.

Allison Peters, a sheriff’s office spokesperson, said that though a lack of testing capability had hampered the jail’s initial response to the virus, the situation has improved.

“As a result of our interventions, cases at the jail have dropped precipitously over the past month,” Peters said in a written statement. “As of yesterday, there were 26 detainees positive for COVID-19 at the jail, and 33 jail staff members currently positive.

“More important,” she added, “virtually all of the new cases in recent weeks have come from newly arrested individuals who tested positive at intake, not from those who were already in custody.”

The authors of the study stress that their main finding is simply a correlation between infection rates and inmate releases at the Zip code level. They did not contact-trace individual infections. Moreover, the relationship between inmate releases and coronavirus spread may be explained by other factors for which they were unable to control.

“Although currently available data are inadequate to establish a clear causal relation,” the authors write, “these provisional findings are consistent with the hypothesis that arrest and jailing practices are augmenting infection rates in highly policed neighborhoods.”

Most strikingly, inmate releases were a much better predictor of Zip code-level infections than other factors studied, such as race, poverty and population density.

The findings may also explain some of the disparities in coronavirus cases and mortality by race. “In light of the well-documented, disproportionate intensity of policing and incarceration in black neighborhoods in the United States,” they write, “the carceral-community spread of disease may bear partial responsibility for the striking racial disparities noted in COVID-19 cases.”

These findings sit precisely at the nexus of the two major crises roiling the country: the coronavirus outbreak and the abuse and mistreatment of black Americans by law enforcement. Many commentators have criticized protesters and public health experts for supporting mass protests against police brutality during a pandemic.

But these findings suggest the two issues are interlinked: black Americans’ higher rates of exposure to the cramped jails and prisons of the criminal justice system may be making them more vulnerable to the effects of the coronavirus.

In the end, that disproportionality affects all of us. “It is possible,” the authors conclude, “that, as arrested individuals are exposed to high-risk spaces for infection in jails and then later released to their communities, the criminal justice system is turning them into potential disease vectors for their families, neighbors, and, ultimately, the general public.”

This story has been updated with comment from the Cook County Sheriff’s Office.