As 12 more Pennsylvania counties prepare Friday to enter the “yellow" stage of Gov. Tom Wolf’s plan to gradually lift coronavirus restrictions, the state’s southeastern corner is far from reaching a needed benchmark. And progress has been the slowest not in Philadelphia, but in its immediate neighbor to the west.

Delaware County has the highest 14-day per capita rate of residents with COVID-19 of all the counties in the Philadelphia region, according to state data. As of Monday, May 18, Delaware County had 263 cases for every 100,000 residents over the last 14 days, slightly above Philadelphia’s 251. But Delaware County’s 14-day total of new cases has stayed steady for about a month, while those of Philadelphia and Bucks, Montgomery, and Chester Counties have dropped.

Local officials and epidemiologists alike say there’s no one clear answer as to why Delaware County has had less success flattening the curve. They point to its concentration of elderly residents, its lack of a dedicated health department unlike the other suburban counties it borders, and its density — the second-highest in the state.

But what is clear is that the county needs to dramatically lower those numbers before starting to reopen. Wolf has said one criterion for counties and municipalities in easing shutdown restrictions is getting below 50 cases per 100,000 population over 14 days.

By Monday evening, officials from the Chester County Health Department, which is serving as a de facto health department for Delaware County during the pandemic, were not able to answer reporters’ questions submitted midday about Delco’s high comparative infection rate.

Of the top four municipalities with the highest concentrations of COVID cases, three border Philadelphia: Yeadon, Darby Borough, and Colwyn. But the second-highest concentration, Media, is in the geographic center of the county.

Municipal leaders, at a loss to explain why their county is an outlier in the region, said nursing homes may be a factor.

Brian Hall, president of Media’s borough council, attributed the higher numbers to Sterling Rehabilitation & Nursing Center, located within the borough. Exact information on Sterling’s cases was not available Monday.

“Media is a small place — the borough is only ¾ of a square mile,” he said. “I think that the fact of having a nursing home in the borough tends to skew the numbers.”

Yeadon’s solicitor, Charles Gibbs, made a similar observation. At 3.1% of its population, the borough has the highest concentration of COVID-19 cases in Delaware County. But it’s also home to two long-term-care facilities — a ManorCare location and Providence Rehabilitation & Healthcare Center at Mercy Fitzgerald — as well as Grace Court, an independent-living apartment complex reserved for senior citizens.

Though Delaware County has a higher rate of nursing home beds per resident than Philadelphia, Montgomery County’s rate is 24% higher than Delco’s, according to federal data.

Delaware County has other demographic and socioeconomic factors that could worsen outbreaks and death rates. It’s the second-most-densely populated county in the state, surpassed only by Philadelphia. Delaware County is home to the 69th Street Transportation Center, a major SEPTA hub. It also has two impoverished municipalities — Chester and Upper Darby — where most residents are minorities. Inequities in access to health care are a longstanding problem for minority groups.

Now, those inequities have been compounded by the virus. Job losses, “essential” jobs that involve exposure to infection, reliance on mass transit, and lack of adequate housing — or no housing — are among the reasons minorities are getting sick and dying at disproportionate rates.

Delaware County’s death rate from COVID-19 is 30% higher than Philadelphia’s.

Michael Levasseur, an epidemiologist and biostatistician at Drexel University, said a likely factor behind Delaware County’s high death rate: it has a higher proportion of old folks, the age group mostly like to develop fatal COVID-19 infections. Sixteen percent of Delco’s population is over age 65, compared with 13% in Philadelphia. Half of Delco’s population is over age 39, compared with age 34 in Philadelphia.

“I think the age disparity explains a lot of" the county’s higher death rate, Levasseur said.

Frail, chronically ill elderly are at greatest risk. Of Delco’s 62 nursing and long-term-care facilities, 40 have had coronavirus outbreaks. The 384 deaths in these places account for 80% of all deaths in the county, according to official data.

Michael Z. Levy, a University of Pennsylvania epidemiologist and biostatistician, said differences in death rates would be even more glaring if they could be analyzed by zip code — say, contrasting the county municipality of Radnor, where census data show the median income is $114,000, with Chester City, where the median income is about $30,000.

Variations in the way counties tally COVID-19 deaths can also muddy the interpretation, said Jeffrey Morris, a Penn professor of biostatistics. The Pennsylvania Health Department has vacillated on whether deaths that are probably but not definitely due to COVID-19 should be added to the death totals of people with positive diagnostic tests for the virus.

On its coronavirus data website, Delaware County says the “unusual jump in the number of Delaware County COVID-19 deaths reported by the PA Department of Health on 5/5/20″ includes a “retrospective review” by hospitals, coroners and health departments.

The review, Morris said, was "presumably to capture other COVID-related deaths” that were not originally counted.

Staff writer Wendy Ruderman contributed to this article.

This story has been updated to reflect that Grace Court is not a long-term care facility for seniors.