More than 250,000 Pennsylvanians who got first doses of a coronavirus vaccine between December and May and have not yet received their second shot will receive a nudge next week from the state Department of Health as it hurries to protect residents from the highly transmissible delta variant.

Of those infected with COVID-19 recently, 99% who died in Pennsylvania and 97% who were hospitalized were unvaccinated, acting Health Secretary Alison Beam said Thursday.

More Pennsylvanians getting vaccinated now is the surest path to avoiding a “worst-case scenario” case surge in the fall, she said — making the next several weeks a make-or-break time for the state’s vaccination effort.

“If you have not been vaccinated yet, now is the time to get vaccinated. And if you’ve only received one dose of the vaccine, it’s never too late to get that second dose,” Beam said. “This is very important because you need to be fully vaccinated to have maximum protection against the delta variant, which is spreading across the country.”

The variant has already caused case surges and overwhelmed hospitals in undervaccinated areas nationwide, leading the Centers for Disease Control and Prevention on Wednesday to recommend fully vaccinated people once again wear masks in public indoor settings if they are in an area with high and substantial transmission or if they have unvaccinated children or adults at home.

» READ MORE: CDC guidance spurs statewide masking in N.J. and calls for vaccination, worry about restrictions in Pa.

The delta variant now makes up about 65% of new cases in Pennsylvania, Beam said, based on CDC statistics for the Mid-Atlantic region. Daily case counts are rising across the commonwealth, with six counties reporting at least 50 new cases per 100,000 people or a positivity rate of at least 8% in the last week, placing them in the CDC’s high or substantial transmission category. The state’s hospitalizations and deaths, as well as most new cases, “are preventable,” she said, urging vaccination.

With the vast majority of people falling seriously ill or being hospitalized with the delta variant unvaccinated, experts say two doses of a vaccine are necessary to protect against the variant. As of Thursday, 62% of Pennsylvania adults were fully vaccinated, a number that has barely budged in more than a month.

This puts the future of the pandemic — and the country’s reopening — in the hands of the unvaccinated and the partially vaccinated.

“We know that we are having a resurgence now of COVID, because we have a reservoir of people who are not vaccinated and so the virus is free to spread in that group of unvaccinated people,” said state acting Physician General Denise Johnson. “That’s why we’re seeing more cases. So the more people that get vaccinated, the less likely we are going to have continued spread [or] the emergence of other variants.”

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

Johnson encouraged Pennsylvanians with questions about the vaccine to talk to their doctors. She also addressed some common concerns: Pregnant women are more likely to have a severe case of the coronavirus and have pregnancy complications as a result, including miscarriage, than they are to have an adverse reaction to the shot, Johnson said. And though people who have recovered from the coronavirus have antibodies, the amount of protection they have varies, so they should still get vaccinated, she said.

The text-message reminder program, announced by Beam on Thursday, mirrors initiatives already implemented in other jurisdictions, such as Philadelphia and New Jersey.

The Pennsylvania Department of Health only started tracking skipped second shots this summer. When officials began matching people’s first doses to their second doses in the state’s data logs earlier this month, they found the state had administered 500,000 fewer coronavirus shots than it had previously reported, but also found that fewer people had skipped their second shot than data previously showed.

» READ MORE: Pa. vaccination doses were overcounted by 500,000 shots, state says

Beam declined to say Thursday how many total Pennsylvanians, including those who got their first shots after May 14, the cutoff date for the current round of text messages, have not completed their inoculations.

Anybody who has received a first dose can get a second shot, no matter how long it has been since the first, Beam said. Residents can get their second dose at any vaccine provider; it does not have to be at the same location as the first shot. The vaccine is readily available at many pharmacies, as well as doctor’s offices, hospitals, and clinics.

“That is our number-one way to have a fall that does not feel like any of the last year,” she said, later adding, “That will get us out of this pandemic.”