Pennsylvania Health Secretary Rachel Levine warned Monday that a cycle seen at the start of the coronavirus pandemic may be repeating: A wave of sickness among younger people that can lead to more severe infections and death among older people.

When the virus first began to circulate in the spring, more people aged 19 to 49 got sick — and then older people fell ill, and the virus spread quickly and killed thousands. Even though Pennsylvania is not seeing the dramatic virus surges plaguing some other states, the number of cases among people under 50 here has risen again, now making up 45% of the commonwealth’s cases, Levine said.

“There are things we can do right now to stop this cycle,” Levine said, speaking at the state’s first formal news conference in several days. “We need to make important choices to lower our risk. We have to adapt our activities to protect against COVID-19.”

In addition to wearing masks and practicing social distancing, Levine asked residents to avoid situations that seem risky — people who show up to a crowded restaurant or a supermarket where others aren’t wearing masks should simply leave. And, she added, “If you are in a situation where you are considering whether or not you need a mask, and you’re thinking about it, then the answer is yes, you need a mask,” she said.

Pennsylvania and New Jersey, which experienced early surges of the virus and remain in the top six states with the highest death tolls, continue to avoid the dire case increases in states such as Florida and Texas — and officials are trying to keep it that way.

California rolled back its reopening plan Monday, closing all indoor dining, all bars, and other indoor operations amid rapidly increasing cases. Florida reported nearly 28,000 new cases on Sunday and Monday alone, with Sunday’s tally breaking the one-day record set in New York in April. The United States has confirmed more than 3.3 million cases, meaning more than one million people got the virus in the last month.

In contrast, New Jersey reported 231 new cases and Pennsylvania reported 328 new cases on Monday.

New Jersey Gov. Phil Murphy said the seven-day rolling average of new cases was declining, and the transmission rate was still below 1.0, which means not every person who gets the virus is infecting another person.

But case counts increased in 43 of Pennsylvania’s 67 counties over the last week, compared with the previous week, and the percentage of tests that are positive was on the rise in 28 counties, Levine said.

Still, the statewide positivity rate has decreased slightly to 4.4%, and Pennsylvania hospitals have plenty of capacity. Levine said state officials were not currently considering moving any counties back to the more restrictive yellow or red phases of reopening.

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Health officials continued to ask people to avoid congregating indoors.

New Jersey officials said some of the spread of the virus there was being driven by indoor gatherings, such as parties at beaches. But Murphy and one of his leading health experts said Monday the state had not seen evidence that people gathering outdoors at beaches — or participating in the large outdoor protests that have occurred in the wake of the death of George Floyd — are causing significant spread of the virus.

“Having said that, if we don’t manage capacity at the beaches, if we can’t get social distancing, we’re probably playing with fire,” Murphy said.

Aside from a few hot spots — including Long Branch Beach in Monmouth County, where officers prohibited access Sunday due to capacity limits — Murphy said compliance with the state’s rules regarding social distancing and masks was “good” in beach areas and on the boardwalk over the weekend.

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The governor cleared NJ Transit buses, trains, and light rail to resume operating at full capacity on Wednesday, lifting the 50% capacity limit on the transit system and private-carrier transit. Face masks are required.

Murphy also said all upcoming special elections will be postponed until Nov. 3, the day of the general election, in order to avoid the public health and logistical challenges of in-person and mail-in voting.

Pennsylvania officials released new statistics: 77% of the state’s confirmed coronavirus patients — 95,742 people so far — have now recovered. Since the start of the outbreak, about 9% of Pennsylvania’s population has been tested for the virus, Levine said.

The state is testing well more than 2% of the population per month and is aiming to test 4% per month and continue increasing that number, she said. More than 1.1 million tests have been performed in Pennsylvania since March.

Some delays in results are occurring because national companies, such as LabCorp and Quest, are being inundated with samples from states across the country. Pennsylvania officials have a call scheduled with those companies to discuss the delays, “but this is a national issue,” Levine said.

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Officials nationwide are watching the positivity rate — the percentage of people whose test results come back positive. That rate is a key indicator of how prevalent the virus is in the community, Levine said. Even when the number of tests being done goes up, that percentage won’t rise unless the spread of the virus is increasing.

Levine said the state considers a positivity rate under 5% to be a “very good sign” and watches rates above 5% very closely. If a rate nears or exceeds 10%, “that’s a bad sign.”

Philadelphia’s rate of positive tests was about 4.1% over the past week. The area of greatest concern in the state is Allegheny County, where officials reported a 9.6% positive test rate on Monday.

Philadelphia recorded 234 new cases from Friday through Sunday. That’s an average of 78 cases per day for the three days, below the city’s recent trend, though weekends often cause delays in reporting.

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Pennsylvania officials remain concerned about the case rises in states including Florida, Texas, South Carolina, Arizona, and California.

“That’s one of the ways that COVID-19 can increase in Pennsylvania — is if a lot of people have come from those hot zones,” Levine said Monday.

Anyone traveling into Pennsylvania or New Jersey from those states and others with high case rates is asked to quarantine for 14 days. Neither state has enforced the directive, and Levine said Pennsylvania had no plans to do so because the state doesn’t track people’s movements.

“You have to make your own choice of whether you’re going to put yourself and your family and your community at risk,” Levine said, “and if we all make the right choices, then we won’t have to worry about our hospital capacity.”

Staff writer Rita Giordano contributed to this article.