Nearly half of all inmates tested at Delaware County’s George W. Hill Correctional Facility over the last two weeks exhibited antibodies suggesting they had been infected with the coronavirus at some point during the pandemic, according to results obtained by The Inquirer.

The findings, like those reported in other correctional facilities that have undertaken widespread testing, showed a much wider entrenchment of the disease behind bars than previously had been detected.

About 12% of the 915 tested still had active infections, according to a report by the GEO Group, the private company that runs the county’s jail.

The company had previously reported, in an April 16 letter to Delaware County’s commissioners, that only 2% of its total inmate population had tested positive throughout the pandemic.

The new results, which cover a testing period that began about a week after that update, show a rate of infection between five and 24 times greater — and up to 50 times that detected to date in Delaware County’s population at large.

“We were very surprised” with the results, said Delaware County Council Member Kevin Madden, who also chairs the Jail Oversight Board. “We’ve been testing people who have been showing symptoms and people who had been exposed. We thought we had a pretty good handle on what this was going to look like.”

GEO officials noted it was impossible to tell whether inmates in the group that showed antibodies were exposed to the virus before or during their incarceration. The majority of those who had active infections were asymptomatic but presumably still capable of spreading the virus.

Those findings underscore the challenges in keeping the coronavirus out of county jails, which have a much higher churn rate in their populations than prisons or other long-term detention facilities, and the risk an outbreak behind bars can pose to surrounding communities.

In a report last week, the Centers for Disease Control and Prevention revised its past guidance and said that testing only symptomatic inmates was “inadequate” to promptly identify and isolate infected people in congregate settings such as jails. And corrections officials nationwide slowly have begun to adopt widespread testing strategies like the one implemented at George W. Hill. New Jersey officials last week announced that they would begin testing all inmates in their state prisons, where 41 have already died from the disease.

Meanwhile, epidemiologists say mass testing results from largely contained environments like prisons and county jails have provided a wealth of information on the behavior of the disease, and just how many people might have contracted the virus without ever developing symptoms.

“Contained populations like jails and nursing homes … give you a microcosm for what the virus is probably doing in society,” said Robert A. Cicchinelli, secretary of the Delaware County Coalition for Prison Reform. “I think if they rounded up a comparable number of people in Delaware County, you’d get comparable numbers.”

Montgomery County, which conducted similar mass testing over two days in mid-April, discovered 30 times as many active cases of the coronavirus among inmates than had previously been detected. Notably, 97% of the 177 with positive test results showed no symptoms of the disease at the time testing occurred, and only three of those became symptomatic later, a county spokesperson said.

GEO did not report how many asymptomatic cases were detected during the course of its mass testing in Delaware County. But a company spokesperson said that such cases constituted “the majority” of the new active infections.

Unlike the Montgomery County tests, which focused only on active infections, George W. Hill also offered serology testing to uncover how many of its inmates and staff had antibodies in their blood suggesting they had already contracted the disease and recovered, or never fell ill in the first place. Antibody tests have been shown to result in more false positives than the swab tests used to identify active cases of the virus.

Of the Delaware County jail’s roughly 1,021 inmate population over the two-week testing period, 108 tested positive for an active infection — up from the reported 22 in mid-April. Antibodies were detected in 385. An additional 133 inmates — roughly 10% — refused the voluntary testing.

Among the 463 staff, 29 tested positive for the coronavirus during the testing period while 112 exhibited antibodies. About 29% declined testing.

“Testing enables us to have more intel on our enemy,” said Madden, the County Council member. “Based on that, the jail has already made changes to the way people are being housed.”

In a statement, GEO said that as a result of the tests’ findings it has separated inmates still susceptible to infection from those who tested positive on either swab or serology tests.