The curve of coronavirus infections is starting to flatten in New Jersey and may be poised to do so in Pennsylvania, making this a “critical time” for social distancing efforts, officials said, as projections Monday showed the two states and New York getting closer to a peak surge.

Gov. Phil Murphy said it was no time to “take our foot off the gas,” and Pennsylvania Health Secretary Rachel Levine warned residents not to get complacent, cautioning it was too early to know whether the rate was truly plateauing. Social distancing, one estimate showed, could make the difference between tens of thousands and a half-million cases in New Jersey.

“Now is the time,” Levine said at a Monday briefing, “to stay home.”

More than 10,000 people in the United States have died of the coronavirus, a grim milestone as experts say the country is heading into what will likely be its most difficult weeks during the pandemic. Nearly half of those deaths have been in New York state.

In Philadelphia, a police lieutenant and two SEPTA employees have died of the virus, it was announced Monday. Fifty-two Philadelphia police officers and 46 city firefighters and paramedics had tested positive for the illness, sources said. Dozens of police officers were self-quarantined, and 140 firefighters and paramedics were awaiting test results.

Officials in Pennsylvania reminded residents to celebrate Passover and Easter virtually and to not gather with anyone outside their households. Delaware set up roadblocks at its borders to question people driving in from Pennsylvania, New Jersey, and Maryland after Gov. John Carney barred all out-of-state residents from entering except for essential business or health-care reasons.

As New Jersey’s death toll passed 1,000, with 41,090 confirmed cases, and Pennsylvania reported 162 deaths and 12,980 cases, the states and their counties continued efforts to secure much-needed supplies and equipment. Chester County announced it would be the first in the state to begin antibody testing, and a mobile hospital was “on its way” to Montgomery County.

Amid continued difficulties in purchasing medical equipment due to nationwide shortages, Gov. Tom Wolf said Monday that Pennsylvania had created a portal to allow state manufacturers and distributors to report their supply chain capabilities and determine how they can collaborate to make coronavirus-related supplies.

Calling on manufacturers to submit their information to the website, Wolf suggested many companies could produce parts for equipment such as ventilators and work together to manufacture needed supplies.

“We’re … working to acquire as much equipment as possible,” Wolf said. “We also need to look inwards… We know there are manufacturers across the commonwealth who are willing and able to help.”

The state recently discovered about 1,000 more ventilators than it had known were available, Levine said, bringing its total to more than 5,000 in hospitals statewide. A purchased shipment of more will be arriving this month, she said.

Philadelphia and Bucks, Montgomery, Chester, and Delaware Counties leaders called on Wolf to implement a policy to allow Southeastern Pennsylvania hospitals to transfer patients to other areas of the state if they become overwhelmed.

Philadelphia Mayor Jim Kenney and the board of commissioners or county council leaders of the four suburban counties said a “patient and ventilator load-balancing” policy would help “ensure no facility is unable to treat patients due to a shortage of necessary supplies, specifically ventilators,” in a letter to Wolf obtained by The Inquirer.

The officials estimated that the five counties were likely to exceed ICU bed capacity by about 11% between Friday and April 20, possibly by 2-1 "in the worst-case scenario,” or more if New York and New Jersey patients come into the region.

But asked about a top health official’s warning over the weekend that Pennsylvania could become a possible “hot spot,” Levine said there was no new data showing a greater danger in Pennsylvania but that it was simply because the state is close to New York and New Jersey.

Dr. Deborah Birx, coordinator of the White House coronavirus task force, said Monday that Americans should only go to the grocery store every two weeks, expanding on her Saturday advice that this was “the moment to not be going to the grocery store, not going to the pharmacy.”

Levine reiterated that people should not go out in public except to get necessary medications or do other life-sustaining activities.

Pennsylvania reported 1,470 new cases on Monday, which along with other recent case numbers could indicate a slowing rate of infection. Wolf and Levine, however, cautioned against reading too much into the numbers at this point.

“It’s way too early to tell whether we’re seeing a true plateau in the numbers of new cases,” said Levine.

State health officials need to assess at least seven days’ worth of data to identify possible trends.

“We are starting to see that the early exponential increase in cases has given way to a much flatter [curve], and so the surge may not be as great as we once anticipated. That’s our fervent hope,” Wolf said.

Montgomery County officials expect the county to hit its peak of cases within the next 10 days. Hospitals there still have open beds, including in the intensive care units, said Montgomery County Commissioners Chair Val Arkoosh, who is a physician with a public health background.

“We actually feel like we are doing OK right now,” she said. “But this is crunch time.”

Chester County was set to become the first in the commonwealth to do testing that shows whether someone has coronavirus antibodies, announcing Monday that it would begin testing for essential workers, including emergency responders, health-care workers, and long-term care facility staff, along with prison staff and inmates.

“Knowledge of who has developed antibodies to the virus can help us tremendously in our strategy to respond to emergencies, treat patients, and care for the elderly,” said Jeanne Casner, director of the Chester County Health Department. The county expects to have 20,000 test kits from a local company by next week.

Murphy said the daily growth rate of new cases in New Jersey had dropped from 24% in late March to 12% Monday. But the models he showed at a briefing were inconclusive, suggesting that the infection rate could peak between April 19 and May 11, with the most hospitalizations likely to occur between Friday and April 28. They also indicated the state could end up with anywhere between 86,000 and 509,000 cases and 9,000 to 36,000 hospitalizations.

Continued dedication to social distancing will prevent the state from reaching the “nightmare scenario” end of those projections, Murphy said.

The crisis remains centered in North Jersey, with 3% of the state’s deaths so far in Burlington, Camden, and Gloucester Counties. Among those three counties, 1,570 new cases were reported Monday, and Camden County reported eight deaths.

The state’s efforts to secure hospital beds will yield 26,000 beds within the coming weeks, state Health Commissioner Judith Persichilli said. Officials in Atlantic and Cape May Counties said they planned to open county testing sites for patients with appointments.

Murphy also signed an executive order Monday allowing retired public sector workers like police officers, nurses and state employees to return to service — to do such things as staff overburdened unemployment offices or provide assistance at police stations and health facilities — without affecting their pension payments.

The drive-through coronavirus testing sites at Citizens Bank Park in South Philadelphia and at Temple University’s Ambler Campus in Montgomery County are set to close on Friday, city and county officials said, as the federal government shut down some sites nationwide that it had assisted in opening.

The city considered keeping the stadium site open with its own resources, but decided to redirect staff and supplies to other venues, Public Health Commissioner Thomas Farley said. He said the site accounted for 10% or less of tests performed in the city.

In New Jersey, the Federal Emergency Management Agency has committed to operating its two mobile testing sites until May 31, Murphy said.

New Jersey Sen. Cory Booker, appearing with Murphy, said he planned to press President Donald Trump and congressional leaders to pass another coronavirus stimulus bill that would include cash payments, financial aid for state and local governments, and provisions for health coverage for the uninsured.

“This is going to be a very challenging time," Booker said. "These coming weeks will be a true test of who we are.”

Staff writers Erin McCarthy, Rob Tornoe, Allison Steele, Susan Snyder, Amy S. Rosenberg, and Tom Avril contributed to this article.