More than one million Pennsylvanians have missed their second coronavirus shots, a no-show rate among the nation’s highest and a gap that could prove critical as the country braces for the spread of the newer, more transmissible Delta variant.

By June’s end, 70% of Pennsylvania adults should have been fully vaccinated. Instead, the state had reached only 60%, meaning many who got their first doses in May skipped the second shots.

With both doses likely needed to protect against the Delta variant — a single shot is only about 33% effective against Delta, one study found — getting the second shot has become more urgent than ever. Yet the share of Pennsylvanians not completing their immunizations has been rising over the last two months, reaching nearly 17% this week, according to an Inquirer analysis of CDC vaccination data.

The reasons so many are skipping or delaying second shots are unclear — particularly since Pennsylvanians have done better than many in other states at showing up for their first shots.

And understanding why could prove difficult: The Pennsylvania Department of Health is not tracking missed second doses, meaning state officials don’t know who skipped the shots or what demographic groups they belong to, information that could be critical for addressing the problem.

» READ MORE: The spread of the COVID-19 Delta variant prompts new restrictions in countries worldwide

Millions are fully vaccinated, but doctors are worried about those who are skipping, particularly as U.S. leaders make it clear that more inoculations are the country’s best hope for quashing Delta’s threat to the pandemic recovery. That has prompted new campaigns here and around the country urging people to return for their second doses.

“If our immunization rates were at 85% or higher level, the individuals who missed the second dose would not be of grave concern,” said Janet Young, chief medical officer for Greater Philadelphia Health Action Inc., which serves patients at community health centers across the city. “But we aren’t even close.”

‘Working in the blind’

The Inquirer’s analysis of CDC data found Pennsylvania ranks fifth from the bottom nationwide in people failing to complete their immunizations. That’s even as it is among the states leading the nation in the number of people with at least one dose — both as a share of the total population and among adults — and with more than six million people fully vaccinated.

» READ MORE: Pa. lifts universal mask mandate with 60% of adults fully vaccinated

New Jersey fared better, with about a 10% or lower skip rate, according to analysis by the state Health Department. The Inquirer’s analysis, which factored in an extra week for people to complete the shots, showed an even better rate.

And officials in the Garden State have been running a program since March, said a spokesperson, that sends texts, emails, and phone calls to people who need their next shots and includes door-to-door canvassing and a social media campaign.

Pennsylvania Department of Health spokesperson Mark O’Neill said the commonwealth does not “specifically track” the number of missed shots but has heard “anecdotally” that some people aren’t showing up.

In an emailed statement Thursday, the department said it is working with the CDC to identify and link recipients of first and second doses and address data challenges, including the difficulty of tracking people who received vaccinations from two different providers or in different states. The department also encouraged all Pennsylvanians to get fully vaccinated.

Pennsylvania’s share of the total population with at least one shot stood at 63% on Thursday, in the top 10 nationwide. More than eight million eligible residents have gotten at least one dose — including, as of this week, three-quarters of its adults. Pennsylvania was among the first states to reach 70% in that metric, the same threshold President Joe Biden set as a nationwide goal.

» READ MORE: COVID-19 cases have plummeted, but vaccinating the holdouts remains a struggle in Philly

But without data about missed second doses, improving the percentages when it comes to people fully vaccinated could be challenging, said Thersa Sweet, a Drexel University epidemiologist.

“You need to find out what populations aren’t getting the second dose, and based on that, you know this is the population we need to target,” she said. “Without knowing who it is, you’re kind of working in the blind.”

Dealing with Delta

Nationwide, about 11% of people who got a first dose didn’t complete their second within four or five weeks of their first doses, CDC data show. The majority of states are performing better than the national average, including New Jersey. Delaware is worse, with nearly 15% not finishing immunizations.

Both the Pfizer and Moderna vaccines require two doses, administered three or four weeks apart, respectively. While people originally were required to get the second shot at the same place they got their first, that’s no longer the case.

People skip second shots for various reasons, health officials around the region said, including not wanting to experience the flulike side effects more often associated with the final dose, mistakenly believing they need only one dose, or missing the appointment and not rescheduling.

» READ MORE: The coronavirus Delta variant is in the Philadelphia area, posing a threat to unvaccinated people

While the first shot of the Moderna or Pfizer vaccines provides significant protection, they are more effective with both shots. One CDC study among health-care workers, done before the Delta variant emerged, found that both were about 82% effective with a single dose and 94% effective with two doses.

But with the Delta variant, the difference is likely much starker: After one dose, the Pfizer vaccine was only 33% effective against the variant in preventing symptomatic disease; after two doses, it was 88% effective, the U.K. study found. (Studies are being conducted to determine how much protection the single-dose Johnson & Johnson vaccine offers against the variant, but a small study this week showed it was effective.)

The U.K. study results have stirred new calls for partially vaccinated Americans to get their second dose. “Get it now,” said CDC Director Rochelle Walensky last week; Anthony Fauci, the government’s top infectious disease expert, said vaccines can “crush the outbreak.”

Leaders in Bucks County, which has a skip rate similar to Pennsylvania, on Wednesday said they would launch a program to contact everyone who has missed a shot. Philadelphia officials are doubling down on a public messaging campaign promoting second shots in response to the “rising threat” of the variant.

Officials say residents who missed their second shot should get it as soon as possible, no matter how much time has passed. “It’s never too late,” said Delaware County medical adviser Lisa O’Mahony. And some of those who skipped an appointment in June, for instance, can likely still get shots within the CDC’s recommended 42-day window.

The Delta variant’s quick spread has already prompted new restrictions in places across the world and has led the World Health Organization, as well as Los Angeles officials, to recommend that even vaccinated people wear masks indoors, in contrast with the CDC’s guidance.

Walensky stressed Wednesday that fully vaccinated people are protected from variants. But for unvaccinated people, as former White House COVID response leader Andy Slavitt said on MSNBC on Thursday, “the Delta variant is a real concern.”

And the more the variant spreads, the more danger there is that other, more virulent variants could take hold, said Sweet, the epidemiologist.

“The more infections that we have, the more opportunities this virus has to mutate,” she said. “People need to think both on a personal protection level but also on a population level. We really, really need to get these viruses to stop circulating in the population.”

» READ MORE: How many people in Pa. got vaccinated and still got COVID-19? The state isn’t counting.

‘Get that second dose’

In Philadelphia and its suburbs, some officials are seeing a higher second-dose return rate than the state as a whole.

Only 4% of Montgomery County residents who got vaccinated through the Office of Public Health’s Pfizer clinics have not shown up for their second dose as recommended, a spokesperson said. And turnout has been high in Delaware County, said O’Mahony, speaking from a second-dose clinic in Yeadon on Monday.

Data tracking by county is trickier because people no longer have to get their second shot at the same place they got their first — as was the procedure earlier in the rollout.

In Philadelphia, for example, 6% to 7% of people who got vaccinated at city sites received only one dose of a two-dose vaccine there in the recommended time frame, spokesperson James Garrow said, but it’s possible those people got one of their doses elsewhere.

The more people who complete their immunizations, Garrow said, “the less likely it is that we’ll see COVID come back and cause the health department to reinstitute restrictions.”

Bucks County’s return rate has been worse, with nearly 18% of people not logged as completing their second doses, according to a county analysis of state vaccination data.

Along with starting a phone bank and considering incentives, officials in Bucks County — where more than 58,000 people eligible for second shots haven’t been recorded as receiving one — are working on a public messaging campaign, said county spokesperson Larry King, to remind residents:

“Regardless of if you are several weeks or months out from your second dose, you still can (and should) go to any vaccine provider to get that second dose,” he said.

Graphics editor John Duchneskie contributed to this article.