With the fast-spreading delta coronavirus variant not yet dominant in the Mid-Atlantic, according to new CDC tracking, Pennsylvania and New Jersey have a window of time to boost vaccinations further to defend against the strain.

But how narrow that window could be depends on how fast the variant spreads: In New Jersey, the share of analyzed cases in which delta was detected rose to nearly 27% last week, up from 16% the previous week, state health officials said, with the percentage of delta cases nearly doubling.

In Pennsylvania, the variant was identified in 3.4% of cases sampled over four weeks through June 19, according to CDC data. Its true presence is likely higher than federal data show, experts say, but Pennsylvania still has had a smaller number of cases detected compared with other states.

And while Philadelphia has not yet seen an overall increase in cases that would signal its prevalence, the more contagious version of the virus has arrived.

“We’re confident saying that there are some cases of delta variant COVID in Philadelphia,” health department spokesperson James Garrow said, “but it’s still rare, so there’s time to get your vaccine.”

Completing those shots is the “most obvious and important” step people concerned about the delta variant can do, and the state’s best defense against the virus’ spread, said Mark O’Neill, a Pennsylvania Department of Health spokesperson.

The world’s attention has turned to the delta variant as it has spread to more than 100 countries, with the World Health Organization this week warning nations about the risks of relaxing restrictions too soon. The strain has threatened the Olympics in Tokyo, which on Thursday barred spectators; extended lockdowns in Australia and London; and led to a third wave in Africa, where vaccine supply is limited.

With the variant now making up more than half of all new cases in the United States, President Joe Biden this week outlined efforts to increase vaccinations and said he would send “surge response” teams into states dealing with outbreaks and low vaccination rates.

» READ MORE: 1 million Pennsylvanians have missed their second shot. They’ll need it against the Delta variant.

It’s unclear whether the variant is more likely to cause severe illness, but it is more transmissible — and both doses of a two-dose vaccine are needed to protect against it, according to early studies. Experts say anyone who is not fully immunized is at risk.

Pennsylvania and New Jersey have higher vaccination rates than the states where the delta variant is gaining strong footholds. The strain makes up less than a third of new cases in the Mid-Atlantic, compared with more than three-quarters of cases in some states in the West and Midwest, according to the CDC numbers released this week.

Still, half of Pennsylvanians and 44% of New Jerseyans are not yet fully immunized. Adding to the urgency: Almost all new cases, hospitalizations, and deaths in recent weeks have been among unvaccinated people — about 97% or more of new cases in May and June in Pennsylvania, New Jersey, and Philadelphia, according to public health data and an Inquirer estimate.

“Now is the time to really dampen the virus’ opportunity to spread,” said Lisa O’Mahony, the medical adviser for Delaware County, where more than half of the population is fully vaccinated but which also includes the city of Chester, where the rate is below 30%. “A lot of people are really concerned about a surge in the fall. With the [vaccination] numbers we have now, there will be pockets that will be susceptible.”

The faster the remaining half of the states’ population gets fully vaccinated, public health officials say, the better chance the region will have at avoiding case spikes or the need for renewed precautions — things that are already happening in other states.

In Missouri, where only 39% of the population is fully vaccinated, a Springfield hospital ran out of ventilators amid a surge of virus patients over the holiday weekend, and in Arkansas, with 34% vaccinated, the governor warned the state was “losing ground” in the effort to end the pandemic. The federal surge response teams, which include experts from the CDC and the Federal Emergency Management Agency, have already been deployed to Colorado and Missouri.

» READ MORE: Receiving both vaccine shots crucial to fighting coronavirus delta variant, study says

And experts say it’s likely the variant is even more prevalent nationwide than CDC data show. The current data don’t capture all new cases because to determine what strain of COVID-19 someone has, a test sample must be analyzed, or sequenced, to decode the virus’ genes — and only some COVID-19 samples are sequenced by the CDC or participating commercial labs.

Health officials in Philadelphia and its suburbs said the sequencing they have seen for the region, while limited, has indicated the delta variant’s level so far remains fairly low; the alpha, or U.K., variant has been more prevalent in recent weeks.

With this juncture in the pandemic potentially critical, state and local officials, like the Biden administration, say they’re continuing to push vaccine outreach efforts. They’re hoping to persuade enough unvaccinated people this summer to give the delta and other variants less opportunity to spread this fall and winter.

They also said it was too soon to know if new measures or guidance, such as the return of indoor masking, would become necessary here and stressed that vaccinated people are the most protected.

“I don’t think fully vaccinated residents need to be worried about the variants at this time,” Bucks County Health Director David Damsker said, “as the risk of serious illness in vaccinated people is still very low.”

In Delaware County, O’Mahony said county workers this summer plan to target undervaccinated areas such as Chester with the help of religious and community leaders and pop-up clinics. And she said they’ll talk with unvaccinated residents about the risks posed to them by the delta variant.

» READ MORE: Nervous about getting the COVID-19 vaccine? Don’t believe these myths.

She hopes enough people will listen in time for the fall — when not only the weather cools but children return to school. She noted that higher community vaccination rates also help protect those who can’t yet get vaccinated, such as young children.

“If we can get our communities to a higher rate of vaccination, it’s really going to change things dramatically for the unimmunized, and we know that’s everybody under 12,” she said. “That’s another consideration to be made. It’d make the schools safer if everybody 12 and older got vaccinated.”

» READ MORE: Pa. and N.J. kids over 12 are lining up for COVID-19 shots as parents experience both relief and hesitancy

The Pennsylvania Department of Health is also working on educating people in areas with low vaccination rates and has been coordinating with organizations sponsoring mobile vaccine clinics, O’Neill said. The newest wave of its public information campaign will focus on the vaccines’ power to protect against variants.

In New Jersey, the state is working with local leaders to reach people through door-to-door canvassing, particularly in areas with low vaccination rates. The strategy has seen success in places such as Ewing and Perth Amboy, and Gov. Phil Murphy begged more residents to heed the message.

“The simple reality is we do not have a pandemic among the vaccinated. This is only now a pandemic among the unvaccinated,” he said Wednesday. “Let’s, please God, end this once and for all.”