Montgomery, Delaware, and Bucks Counties are now seeing substantial transmission of the coronavirus, an increase spurred by the fast-moving delta variant and one that means most of the region now falls under federal recommendations to wear masks in public indoor places.

The Centers for Disease Control and Prevention on Monday reclassified the infection rate in those counties from “moderate” to “substantial,” bringing the entire region except Chester County into the elevated category.

The CDC’s nationwide masking recommendation, announced last week, is based on counties’ transmission levels and applies only to those with substantial or high spread of the virus. Philadelphia moved into the substantial transmission category over the weekend but had already recommended masking indoors.

The delta variant now makes up the majority of new cases in Pennsylvania, New Jersey, and the U.S., but the levels of transmission in the Philadelphia region remain much lower than in many places across the country seeing major outbreaks.

Chester County is the only county in the region where spread remains at a moderate level. Gloucester and Camden Counties also have seen substantial transmission, joining Burlington. The designations, based on new cases in the last seven days, are updated by the CDC daily, meaning counties can move in and out of transmission categories quickly as the level of spread changes.

As of Monday, Pennsylvania had 25 counties with substantial or high transmission, meaning they had a case rate of at least 50 new cases per 100,000 people or a test positivity rate of 8% or higher from Monday through Sunday. When the CDC announced the masking recommendation last Tuesday, only seven of the state’s counties were in those categories.

All but one of New Jersey’s counties had substantial or high transmission as of Monday, also an increase from last week. While delta has spread more slowly in the Northeast — states with higher vaccination rates — than less-vaccinated states in the South and elsewhere, New Jersey Gov. Phil Murphy warned that the variant was “working its way across our state” and definitively causing cases and hospitalizations to rise.

Last week, he announced more than 400 new COVID-19 hospitalizations at his weekly briefing; this week, it was more than 500. Almost all the cases are of people who are unvaccinated or not fully vaccinated.

“My guess is this gets worse before it gets better,” Murphy said Monday, urging residents to get shots to increase their communities’ level of protection — and making, for the first time since the vaccines became available, a prediction that echoed his briefings last year as the virus took hold.

The spread of the delta variant and the evolving body of knowledge about how it affects vaccinated and unvaccinated people has shifted the landscape rapidly in recent days, prompting renewed calls for masking and adding even more urgency to the push for vaccinations.

» READ MORE: The delta variant may spread as fast as chickenpox, and other facts about new CDC data

New data from the CDC reported Friday indicates that the variant spreads as easily as chicken pox, and, unlike with earlier strains, vaccinated people may be as likely to spread it as unvaccinated people. But that doesn’t mean the vaccine is ineffective. Vaccinated people are far less likely to get very sick, be hospitalized, or die.

That leaves questions open as scientists study the emerging threat. “We’re still getting our arms around the degree to which people who are vaccinated are vulnerable to delta,” said Esther Chernak, a physician and director of Drexel University’s Center for Public Health Readiness and Communication.

But doctors say one thing is certain: Increasing the vaccination rate now and tamping down the virus’ opportunities to spread and mutate into forms that may be more resistant to the vaccine — as it did with the delta variant — is key to suppressing the pandemic.

“Since the delta variant is more infectious, you need to have a higher percentage of people who are immune to keep it from spreading,” said New Jersey Medical Director Edward Lifshitz.

From relief to anxiety

The rise of the variant is a turnabout for places with relatively high vaccination rates — including New Jersey and Pennsylvania, the seventh- and eighth-most vaccinated states, respectively, as of Monday — and spots in the Philadelphia region like Montgomery County, which has almost 60% of its eligible residents fully vaccinated but still has hundreds of thousands to go.

As of Monday, Delaware County’s rate of new cases for the last seven days was 55 new cases per 100,000 people; Bucks County’s was 56; Philadelphia’s was 52; and Montgomery County’s was 60, according to the CDC.

Less than two months ago, Montgomery County commissioners’ chair Val Arkoosh said, she was finally relaxing as vaccinations increased and coronavirus case counts plummeted there — but now, anxiety is creeping in again.

Residents are feeling it, too. The developments have contributed to uncertainty among vaccinated individuals about the risk that the new variant poses to themselves and their loved ones. In Chester and Montgomery Counties, officials say that more people seem to have been masking up in indoor public settings such as grocery stores.

And both counties have seen recent upticks in vaccinations, though not “in droves” as at previous points in the rollout, Arkoosh said in an interview last week before Montgomery County entered the CDC’s substantial transmission category.

Though it’s now known that vaccinated people can spread the delta strain and some are becoming infected, most outbreaks are still occurring among people who are unvaccinated.

In New Jersey, for instance, 0.13% of all vaccinated people have tested positive for the virus, and 95% of current hospitalized patients are unvaccinated or not fully vaccinated, Murphy said. Of Pennsylvanians infected with the coronavirus recently, 99% who died and 97% who were hospitalized were unvaccinated, acting Health Secretary Alison Beam said late last week.

“I think that the highly vaccinated regions of our country and our state are pretty protected,” said Drexel’s Chernak. “If you look at the counties where cases are surging and hospitalizations are rising, they’re in places where vaccination is well under 50%. We’re not there.”

She said the “least risky” social activity for vaccinated people is hanging out with other vaccinated people, while the most risky would be attending a maskless indoor gathering with many people of unknown vaccination status.

And if they’re going into an indoor business with others who may not be vaccinated, it’d be smart to follow the CDC recommendation and mask up.

Vaccinated people are “just going to have to be patient and do what they always do: Listen. [Like] when they went and got vaccinated, when they [first] put on the mask,” said Herb Conaway, a physician and director of the Burlington County Health Department.

He said their actions can still influence others, too: “Maybe there’s somebody that decides I’m going to put a mask on if another guy is doing it.”

Race against delta

On Monday, New Jersey announced it will require vaccinations or weekly coronavirus testing for some state health workers, and possibly mandate the shots in the future if rates don’t increase. This week, Pennsylvania is set to send second-dose reminder texts to more than 250,000 residents who never completed their vaccinations. Experts say both doses of a two-dose vaccine are key to getting the most protection.

» READ MORE: New Jersey to require health workers to be vaccinated or undergo weekly COVID-19 testing

“The pool of the most vulnerable and exposed shrinks every day” as more people get vaccinated, Murphy said. “The pool continues to shrink. [But] the variant is probably more aggressive in its pace than the pace of the pool shrinking.”

Even Chester County, which is among Pennsylvania’s most vaccinated counties and the region’s only county without substantial transmission, sees room to improve. While more than 70% of eligible residents have received at least one vaccine dose, county health director Jeanne Franklin said she won’t be satisfied until nearly 100% of them have had both their shots.

“Yes, there has been a direct correlation between the lowering of our rate of transmission with the increase in the number of people being vaccinated,” she said. “But we are not in a bubble, and we monitor our rate of transmission daily, to assess the trends, and the right public health messages that go with those trends.”