In Delaware County, residents were phoning the county’s COVID-19 hotline, asking if their vaccination would protect them against the new variant. Chester County was anticipating more people seeking boosters, or even their first immunizations. New Jersey health officials were communicating with laboratories across the state, asking them to be on the lookout for the mutation.

With the public’s attention captured by the emerging coronavirus variant known as omicron, officials in the region on Monday urged renewed vigilance in mask-wearing and doubled down on their calls to get vaccinated and take booster shots. But their reasons to do so were deeper than fears that the new variant would soon arrive: In Philadelphia, at least, the delta variant is still spreading, and faster than it had been.

“On Friday, our average new cases per day was as high as it was in early September,” said health department spokesperson James Garrow. “It’s smart for people to keep an eye out for updates on omicron, but to take precautions against the delta variant.”

With scientists worldwide racing to learn more about the new and relatively unknown variant, including how transmissible it is and how well the existing vaccines protect against it, officials said residents should be prepared for omicron to arrive in the United States.

New Jersey Gov. Phil Murphy and his top health officials said they were watching the research and monitoring the global spread of omicron. But with so much unknown, they said the public should expect a period of uncertainty for the next few weeks.

“Do not get hysterical. That is not warranted. We just don’t know enough,” Murphy said. “Folks, I would say: Accept that there’s going to be an uncomfortable period of time here where it’s still delta driving cases … and we’re going to have to allow the research” on the new variant to unfold.

Pennsylvania health officials said they were sequencing COVID-19 test specimens and encouraging vaccinations.

“We need Pennsylvanians to get vaccinated,” a Department of Health spokesperson said, adding that those eligible “need to use this time to get their boosters.”

Meanwhile, concern was multiplying worldwide. The World Health Organization warned that the variant poses a “very high” global risk. As some countries closed their borders — a step South Africa and the World Health Organization criticized — others reported cases. None had been reported in the United States by Monday, but Canada identified the first cases in North America.

Moderna and BioNTech both said Monday they were looking at tweaking their vaccines if necessary; Moderna CEO Stephane Bancel said in an interview on CNBC that the company anticipates vaccines will lose efficacy against omicron but that it will take about two weeks to know more definitively.

» READ MORE: Some states know a lot about vaccinated people who have gotten COVID-19. Pennsylvania doesn’t.

Locally, the region’s health departments were monitoring the situation, awaiting more information on transmissibility, severity, and vaccine effectiveness.

“If there is no significant change in any of these parameters, then we will proceed on our current course with mitigation strategies and immunization,” said Richard Lorraine, Montgomery County’s medical director.

At Penn Medicine, where a research center sequences COVID-19 samples drawn from positive COVID tests in an effort to understand how the virus is circulating and evolving, experts stood ready to identify and sequence the virus then share that information with labs throughout the world.

“I would expect we’ll see it in weeks, but I could be wrong,” said Frederic Bushman, chair of microbiology at University of Pennsylvania’s Perelman School of Medicine and co-director of the center.

Delaware County medical adviser Lisa O’Mahony, a physician, said she expects vaccines to still provide some protection. For example, she said, even if one year’s flu shot isn’t matched perfectly to a dominant strain, it still reduces the likelihood of infection and makes people feel less sick if they do get the flu.

In addition to getting vaccinated, public health officials here urged the public to take precautions such as wearing masks and avoiding crowded indoor places.

Delta remains the dominant variant worldwide, including in the United States, where cases had increased 25% in the two weeks leading up to Thanksgiving. Cases and hospitalizations have been on the rise in Pennsylvania and New Jersey, though hospitals say they have room for the growth in patients.

With most mitigation measures lifted for months, public health leaders had been pushing for higher vaccination rates to ward off a potential winter surge, particularly with the arrival of the holiday season. News of the omicron variant strengthened their message.

“It is as sure as the sunrise that we’re going to see this omicron here in the United States in the near future, and people really need to strengthen their determination to get fully vaccinated … [including] parents getting their 5-and-ups in,” said Burlington County Health Department Director Herb Conaway. “It should increase the impetus individuals have to get vaccinated. And I hope for the vaccine-hesitant, this news will cause them to get off the fence and get into the pool of vaccinated persons.”

» READ MORE: COVID-19 vaccine clinics are busy again as people line up for pediatric shots and boosters in Pa. and N.J.

Echoing national experts, he and other local officials also pushed boosters as a way for people to prepare for the variant’s arrival.

“Increasing our booster uptake will be vital in blunting the impact of this and any other new variant that may occur,” said New Jersey Health Commissioner Judith Persichilli.

The new variant has a large number of mutations, many more than the delta variant had. As viruses spread — as the coronavirus has continued to do because much of the global population remains unvaccinated — they can mutate, creating new strains. That means vaccines could be less effective, because they were not formulated for the variant. At the same time, variants may be more, equally, or even less infectious as previous strains.

That information was impossible to know on Monday. But what has remained certain is that getting the world’s population vaccinated is the way to cut down the virus’ opportunities to mutate and, thus, prevent more variants.

“The way to minimize the increase of mutations and the emergence of more variants is to get vaccinated,” said New Jersey State Epidemiologist Christina Tan. “If you get vaccinated, you’ll do your part in terms of trying to keep down the spread and the emergence of new variants.”

Murphy, who received his booster shot Sunday and said he was “feeling great,” said there remained many questions and few answers about omicron.

“The importance of preventing other spread of delta or, we hope, to prevent omicron from gaining a foothold is tantamount,” he said. “By the millions, you’ve all done so much the right thing, and we’ll continue to do that and get through this together.”

Staff writer Jason Laughlin contributed to this article.