The curve of coronavirus infection is flattening in Pennsylvania and New Jersey, officials said Monday. Social distancing is working, they said — and has helped to avert a worst-case scenario in New Jersey hospitals — but progress is fragile.

Lifting the measures too soon could set the states back and reignite the virus’ spread, a coalition of governors including Pennsylvania’s Tom Wolf and New Jersey’s Phil Murphy warned Monday. They announced a multistate council with other governors in the Northeastern United States to create plans for returning to a more normal life as President Donald Trump claimed “total” authority to reopen the country.

Pennsylvania’s case count has plateaued thanks to residents and business owners following the state’s school and business closures and stay-at-home orders, Health Secretary Rachel Levine said.

Because of social distancing, the state has avoided a “much, much worse” death toll to date, she said. Philadelphia officials indicated the same, saying that while city hospitals were busy, there remained “plenty” of vacant beds and available ventilators.

“This has been a very difficult and hard sacrifice for Pennsylvanians, but the sacrifice is working,” Levine said. “The closures are saving lives in Pennsylvania. If we stop those efforts now, our health systems will become overwhelmed, and then more will be lost to this dangerous virus.”

In an opinion issued Monday, the state Supreme Court upheld Wolf’s March 19 order for non-"life-sustaining" businesses to shut down, saying he used the state’s emergency code and the governor’s police powers appropriately.

In New Jersey, officials had feared that the past weekend would bring a surge in cases and up to 14,400 people hospitalized. Instead, there was good news: an “undeniably flattening” curve that Murphy said was the result of good social distancing. As of Monday, 7,781 residents were hospitalized with the virus.

“It could have been a lot worse,” Murphy said. “We’ve already probably dodged a significant chunk of this bullet.”

Nationally, the number of new cases appears to be stabilizing, said Robert Redfield, director of the Centers for Disease Control and Prevention. He was cautiously optimistic that the United States was nearing the peak of the pandemic, as the number of cases passed 500,000 over the weekend.

As a fierce and destructive storm swept through the region Monday, officials cautioned that the “great news” that the rates of infection are slowing in different areas does not mean the region is out of the woods.

If New Jersey “takes its foot off the gas,” Murphy said, it could at worst see up to 36,000 residents hospitalized and 6,000 on ventilators, according to projections from the governor’s Office of Innovation. The shutdown measures in place are an effort to make sure those projections don’t come true.

“Our job is to make sure we pound the curve from a worst-case scenario to a best-case scenario,” Murphy said.

New Jersey now has 64,584 residents who have tested positive for the disease, while the state’s death toll has reached 2,443. The majority are in North Jersey. In South Jersey, there are more than 3,000 confirmed cases in Burlington, Camden, and Gloucester Counties. Camden County officials reported 10 deaths on Monday, the most there in a single day.

And the peak number of cases is yet to come in Pennsylvania, Levine said. With 1,366 newly reported cases Monday, the state has 24,199 confirmed cases and 524 deaths. Of people who have tested positive for the virus, nearly 1,200 are health-care workers and nearly 1,700 are residents of long-term-care facilities.

A 67-year-old Philadelphia man serving a life sentence at SCI Phoenix in Montgomery County became the first state prison inmate to die from complications of the virus. And Kevin Bundy, a 33-year-old Delaware County paramedic, died after contracting the virus at work, county officials said.

Numbers indicate the pandemic is beginning to peak in Philadelphia, where 427 new cases and 14 deaths were reported Monday, said city Health Commissioner Thomas Farley. The new numbers continued a slowdown in cases. Though hospitals are still well positioned to handle the flow of patients, Farley cautioned, “We’re not there yet. We’re just showing signs we may be stabilizing. We’re not on the other side of the epidemic yet.”

The medical station at the closed Glen Mills Schools will be relocated to East Stroudsburg as Delaware County officials say they are “prepared” and can “handle the capacity of patients they are receiving and anticipate receiving” due to the coronavirus, county officials said Monday night. Members of the Pennsylvania Department of Health and National Guard will begin breaking down the site Tuesday morning and then will relocate it to an undisclosed location in East Stroudsburg.

Preparations were still underway for overwhelmed hospitals. Both Pennsylvania and New Jersey announced triage guidelines for hospitals that lay out procedures for prioritizing which patients get care if the system is overwhelmed. The policy is meant to do the “greatest good for the greatest number,” Health Commissioner Judith M. Persichilli said.

And more sorely needed medical equipment was on its way. Vice President Mike Pence said the federal government would send Pennsylvania two million N95 masks. New Jersey also received 200 more ventilators from the federal stockpile, Murphy said, and the government of Taiwan is donating 300,000 medical masks to the state.

Grocery stores continued expanding safety measures in Pennsylvania, with the Fresh Market requiring customers to wear masks beginning Tuesday and Giant joining Acme, Aldi, and other supermarkets in limiting the number of shoppers allowed at any one time. New Jerseyans entered their first week under a new mandate for stores to limit capacity and require all shoppers and workers to wear masks.

Murphy, Wolf, and the governors of New York, Delaware, Connecticut, and Rhode Island on Monday announced a joint effort to create a plan for the eventual slow and safe reopening of the region. Leaders of the three West Coast states announced similar collaboration — hours after Trump asserted in a tweet that he would be the one to decide when the country relaxes its social distancing guidelines, not individual governors.

“Well, seeing as we had the responsibility for closing the state down,” Wolf said on an afternoon call with reporters and the other governors, “I think we probably have the primary responsibility for opening it up.”

At a Monday evening briefing, a testy Trump, who has weighed reopening parts of the economy as early as next month, said his authority could overrule governors’ stay-at-home orders. “When someone is president of the United States, the authority is total,” he said.

The states and local officials have constitutional responsibility for public safety.

“This is about being smart,” New York Gov. Andrew Cuomo said. “Not political — smart.”

The six governors said their group of public health and economics officials would begin working Tuesday to devise a plan for reopening the region’s economy. Massachusetts, the only state in the group with a Republican governor, became the seventh state to join late Monday.

The multistate council will use public health and economic data to guide governors to “safely” bring life back to normal in the Northeastern U.S. without putting the public at risk of another spike in infections.

Determining when it’s safe to begin easing restrictions will include looking for consistent declining trends in the number of cases, and daily increases and percentage of positive tests, Levine said. Any reopening plan in Pennsylvania will be gradual, with officials watching for outbreaks as different sectors slowly come back to life, she said.

The governors said it was key to synchronize their approaches and to only reopen after public health experts and health recovery data indicate it’s appropriate.

“You’ve got to get people healthy first, and then you can reopen the economy,” Wolf said.

Staff writers Jeremy Roebuck, Ellie Rushing, Allison Steele, Rob Tornoe, and Sean Collins Walsh contributed to this article.