Pennsylvania has become the 10th state to see 70% of its adults with at least one coronavirus vaccine dose — offering a promising indicator for suppressing the virus spread and putting the state on track to lift its mask mandate by the end of June, officials said Thursday.

Hitting that benchmark, a goal President Joe Biden set for the nation to reach by July 4, puts Pennsylvania’s vaccination rate in the top 10 nationwide and means 70% of the state’s adults should be fully vaccinated by the end of next month.

But the progress isn’t uniform: In 26 counties, more than 60% of residents haven’t gotten even one shot, according to state data through Tuesday. In some, it’s below 30%. Even Philadelphia slightly lags its neighboring counties, with more than half of all city residents still without a shot, adding urgency to ongoing efforts combating hesitancy and to the development of vaccine incentives.

The slower pace of vaccination in rural counties likely won’t threaten the state’s prospects for mass immunity, experts said, but could lead to small COVID-19 outbreaks that would be otherwise preventable.

Despite the muted interest in north, central, and northwestern parts of the state, experts say overall signs are good as the state approaches a full reopening on Monday.

“I’m actually optimistic that we will get to a good point in Pennsylvania, likely enough to prevent exponential spread from happening again,” said Jeffrey Morris, a University of Pennsylvania biostatistician who has tracked data throughout the pandemic. “I’m very, very encouraged with where we’re going.”

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Scientists and doctors have identified 70% as an approximate threshold for reaching mass immunity, or having enough people protected from the virus that it’s unable to spread widely or cause major outbreaks. The exact percentage needed is unknown, but experts said case rates are already dropping, thanks to vaccinations, and should improve by the end of June.

The number of people fully vaccinated statewide is expected to soar in the coming weeks as people get the second shot in their two-dose regimen. In Chester County, for instance, 37% of all residents are fully vaccinated and health officials anticipate that 66% will be by July 1, including 84% of the county’s adults.

But other counties look unlikely to come close to 70%. Some pockets of the commonwealth lag far behind in vaccinations as demand remains low in rural areas. Bedford County, for instance, had the third-highest average rate of new cases this week — 19 per 100,000 people — and only 28% of its residents partially or fully vaccinated.

“I would say very few of us were prepared for these numbers,” said Michael Scalzone, chief quality officer of Guthrie Clinic in Bradford County, where 29% of residents have gotten at least one shot.

To the east, a spokesperson at the Lehigh Valley Health Network said a 70% vaccination rate “looks like it will be difficult to achieve” in the seven central-eastern counties it covers. And Fulton County, which has the lowest vaccination rate in the state, fell behind due to a low vaccine supply and lack of providers, a Fulton County Medical Center spokesperson said.

That may not affect the statewide or nationwide goals of suppressing the spread of the virus — “You’re not going to see too much huge outbreak and spread from the very rural areas,” said David Damsker, the Bucks County health director — but it will likely still have consequences.

“The idea is that you’re not going to have a forest fire burning out of control. You may have some local brush fires,” said Morris, the Penn researcher. He noted that widespread vaccination provides some protection to people in unvaccinated areas by decreasing the chances of spread and exposure.

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But for individuals, the virus remains dangerous. Abby Rudolph, infectious disease and social epidemiologist and associate professor at Temple University, noted that outbreaks may not become “catastrophic,” but may still occur particularly among social groups that are largely unvaccinated, since people tend to associate with people who share their views.

Most Pennsylvanians who have recently gotten seriously ill or died of the coronavirus were not vaccinated, acting Health Secretary Alison Beam said Thursday, and of the five counties seeing substantial community spread this week, three were among the bottom 10 for the percentage of people with at least one vaccine dose.

That’s even as case numbers drop historically across the state — Pennsylvania reported fewer than 1,000 new cases Thursday for the first time since Oct. 5.

“Whenever you see someone who’s on a ventilator and they weren’t vaccinated, you think, hey. ... Just by getting those people vaccinated, they wouldn’t be on a ventilator today, most likely,” Damsker said, adding that most of the current coronavirus patients in Bucks County’s hospitals had not been vaccinated.

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Experts say the last several months have made it clear that the vaccine is effective and has helped drive down case counts in Pennsylvania, the U.S., and other countries, as well as in isolated communities such as nursing homes.

New Jersey, which has nearly 73% percent of adults with at least one shot and 48% of all residents fully vaccinated, according to CDC data, is among the 10 states that have reached Biden’s goal. All are in the Northeast except New Mexico and Hawaii.

While Pennsylvania tied lifting its mask mandate to the percentage of vaccinated adults, the total number of vaccinated people is also a key metric. Of the state’s entire population, 57% have been partially or fully vaccinated. That number is 49% in Delaware County, 60% in Montgomery County, and 51% in Bucks County, according to state data as of Tuesday. In Philadelphia, 45% of all residents have received at least one dose, according to city statistics.

“Once we get from 50% to 60% fully vaccinated in the adult population [in Pennsylvania], I think we’re going to see a further improvement in the cases,” Morris said. “That’s a big number I’m really hoping we reach, and if we can get to 70%, then I feel really good at that point.”

Data journalist Chris A. Williams contributed to this article.