Coronavirus cases have been rising across the Philadelphia region since June, but Delaware County is experiencing a particularly sharp increase in new cases.

The county has gone from an average of 15.7 new cases a day on June 27 to 78.3 new cases a day on Tuesday — an increase of 398%, according to an Inquirer analysis of Pennsylvania Department of Health data. Chester, Montgomery, and Philadelphia Counties also are experiencing a rising 14-day average of new cases, according to the analysis.

Delaware County was one of three in the state — along with Philadelphia and Allegheny Counties — to see days with more than 100 new cases in recent weeks.

“We are concerned with what we have seen from Delaware County, amid a resurgence of cases in the southeast,” Nate Wardle, a spokesperson for the state Health Department, said in a statement. “As with most places across the state, we are seeing increases in COVID-19 cases because of community spread.”

Jeanne Casner, director of the Chester County Health Department, attributed the increase in Delaware County to travel, particularly to beaches, and socializing without masks. Chester County is conducting contact tracing and overseeing COVID-19 outreach in Delaware County, which does not have its own health department.

She said the county’s contact tracers have not identified any specific incidents or events that attributed to the increase in cases.

Casner’s comments echoed Philadelphia Health Commissioner Thomas Farley, who earlier this week said that “the epidemic is likely to be worse before it gets better,” when addressing the city’s rising case count.

Like other counties, Delaware County has seen concentrations of COVID-19 cases in towns with nursing homes, where older adults with fragile immune systems and multiple chronic conditions have been particularly vulnerable to the virus. Towns with strong minority communities, such as Yeadon, have also struggled with outbreaks.

Nationally data show Black people are more likely to experience severe cases of COVID-19 in part because they experience higher rates of the underlying medical conditions that make them vulnerable to the virus. In Philadelphia, for instance, Black patients account for about 60% of COVID-19 hospitalizations, while just over 40% of the city’s population identifies as Black.

After a steady decline in cases, infections began rising again in the Philadelphia region in late June, according to an Inquirer analysis.

Residents became more brazen in venturing out of their homes, as businesses reopened, restaurants welcomed back diners and impromptu summer vacations were put back on the calendar.

Epidemiologists long predicted that as states suspended their lockdowns, the number of coronavirus cases would rise again, which is why local and state health officials have urged people to continue to practice social distancing, wear a mask, and avoid large gatherings.

“Even small case increases are concerning and are being watched very carefully,” Wardle said.