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

Pennsylvania is not tracking all coronavirus cases in vaccinated residents, leaving the state without data that can help demonstrate vaccine efficacy, detect the spread of variants, and fully inform unvaccinated people of the risks they may face.

As other states report data that indicate nearly all their cases now are found in unvaccinated residents, the Pennsylvania Department of Health told The Inquirer it could not say what proportion of new coronavirus infections has occurred in those who aren’t immunized or what share of people hospitalized with the virus had been vaccinated.

The U.S. Centers for Disease Control and Prevention is tracking only hospitalizations and deaths nationally, leaving it up to states to analyze cases. New Jersey and Delaware are among those that are, as is Philadelphia, which runs its pandemic response independently of the state.

In all three places, more than 98% of recent coronavirus cases have been in unvaccinated people. The findings support a growing national body of data that, six months into the U.S. vaccination effort, suggests nearly all severe coronavirus cases could be prevented if the whole country got vaccinated.

Other states — including Colorado, Arizona, Minnesota, Indiana, and Washington — have also tracked the data and found that very few fully vaccinated people are testing positive.

The information could be key to persuading people who don’t feel an urgency to get vaccinated, experts said, which is particularly important as public health officials nationwide push to get Americans inoculated this summer, before risks rise in fall and winter.

“That’s the million-dollar question: What’s the message that’s going to convince the vaccine-hesitant or vaccine-reluctant person to get vaccinated?” said Esther Chernak, a physician and director of Drexel University’s Center for Public Health Readiness and Communication. “In my mind, [the low percentage of cases in vaccinated people is] a really compelling argument.”

Tracking all cases among inoculated people is also key to assessing the pandemic’s status and how well the vaccine is protecting people in the real world against the virus and its variants. Not tracing minor and asymptomatic cases in addition to the severe cases that lead to hospitalization or death could conceal signs of variant infections — such as the new, more transmissible delta variant spreading across the U.S.

And while extensive contact tracing was generally impossible at the height of the pandemic, when states were recording thousands of new cases a day, most states should now be able to do the kind of tracing or analysis New Jersey, Delaware, and others are doing, said Chernak. Four public health experts who spoke to The Inquirer agreed that so-called breakthrough cases — those in vaccinated people — should be tracked by the states or CDC.

“It’s essential for people who are unvaccinated to really see the data [and] at how high a risk they are of contracting COVID right now,” said University of Delaware epidemiologist Jennifer Horney. “We have 95% or more of cases in most states being in unvaccinated people. ... We need to tell this story.”

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Pennsylvania does report hospitalizations and deaths of vaccinated people to the CDC as required. (The state doesn’t have the total number of coronavirus patients hospitalized for the same time period, however, so can’t calculate what share of them were vaccinated.)

Its reported numbers provide evidence of the same level of vaccine efficacy seen in data from other states, showing that a minuscule fraction of immunized residents have died or been hospitalized from the virus.

Pennsylvania’s breakthrough case numbers are most likely similar to other states, said University of Pennsylvania biostatistician Jeff Morris. An Inquirer data analysis based on an assumed 90% effectiveness rate for full vaccination estimated that 3% of cases in Pennsylvania from March 5 to June 21 were among fully vaccinated people.

Asked about breakthrough case data, Pennsylvania Department of Health spokesperson Mark O’Neill said officials were focused on providing other data the department has routinely published throughout the pandemic and on making the vaccine accessible to residents.

He also said that, like many other states, the commonwealth’s vaccination and hospitalization data are not linked, which means determining how many coronavirus patients are immunized would be labor-intensive.

The Department of Health anticipates analyzing vaccination status and COVID-19 cases later in the year, O’Neill said. In addition, the department is reviewing whether it would be possible to implement a new requirement for hospitals to report vaccination status of coronavirus patients.

As President Joe Biden expects to miss his goal of getting 70% of American adults at least partially vaccinated by July 4 — a threshold billed as a major step toward putting the pandemic in the rearview mirror — millions of Americans are still unvaccinated.

And making sure people get both doses has become a more urgent priority: While one dose of the two-shot Moderna and Pfizer vaccines confers significant protection against most COVID-19 strains, data so far suggest both doses are needed to protect against the growing delta variant.

Millions in the Mid-Atlantic still need to be immunized, though Pennsylvania and New Jersey rank in the top 10 nationwide for giving residents at least one dose. Both have more than 70% of adults with at least one shot, and Delaware stands at 69%, according to the CDC.

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With the country trying to get as many people fully vaccinated as possible, the real-life data on cases in New Jersey and Delaware, as well as the hospitalization and death data available for all states, highlight just how effective vaccines are.

Among the most compelling numbers are the rates of breakthrough cases among the states’ vaccinated populations, a slightly different measure than the proportion of cases among immunized people out of total reported cases.

In both New Jersey and Delaware, less than 1% of all vaccinated residents — about 3,500 people across both states combined — later tested positive for the virus, according to data provided to The Inquirer.

While New Jersey’s breakthrough case rate remains low, 9% of all cases between April 23 and June 14 were in vaccinated people, a small increase from previous months — though still less than a percent of the state’s immunized residents. Health Department spokesperson Donna Leusner said that kind of uptick “is to be expected as people have been vaccinated longer — thus [a] longer period in which they could have been exposed.”

Across the U.S., less than 1% of vaccinated people have been hospitalized with or killed by the virus, the CDC has found. Of more than 144 million vaccinated Americans, 3,729 had been hospitalized or died from the coronavirus as of June 14.

Other studies have shown similar results, such as one by the Cleveland Clinic that found more than 99% of its coronavirus hospitalizations from January through April weren’t fully vaccinated.

In Pennsylvania, vaccinated Pennsylvanians have made up 1% or less of coronavirus deaths since Feb. 22. All but 37 of the 3,468 people who died after testing positive for the coronavirus in the last four months were unvaccinated, state data show, and some of those 37 may have died of other causes.

In New Jersey, the proportion is even smaller, with 31 of the state’s 7,274 deaths recorded between Dec. 15 and June 14. That’s 0.4%.

And hospitalization reports indicate the same: As of June 14, 67 vaccinated New Jerseyans had been hospitalized for COVID-19, and in Delaware, the number was 18 as of June 11. Pennsylvania has seen 257 since the end of February, but that number includes people who were hospitalized for other reasons but tested positive.

“Not just the cases that we’re seeing are really driven by the unvaccinated people, but even more so the hospitalizations,” said Morris. “And that’s a key factor in terms of disease management. … It’s unlikely that we’re going to eradicate [COVID-19], but we want to turn it into ... [a disease that’s] not something that puts people in the hospital, that threatens their life.”

Various variables can skew the counts slightly — particularly for case counts, which likely do not include vaccinated people who contract asymptomatic or mild cases they never knew they had.

But when states like Pennsylvania don’t track the information, experts said, unvaccinated people don’t know their true risk. Only an educated guess is possible, using the number of unvaccinated people and assuming a vaccine-efficacy rate based on data from clinical trials of all three vaccines.

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Using these assumptions, for example, the Inquirer data analysis found that the case rate for unvaccinated people in Pennsylvania is likely higher than the overall case rate among all residents, vaccinated and unvaccinated.

“There’s a misunderstanding when we present data that’s for the total population, and actually a large share of that population is not at risk,” Horney said. “That waters down the risk and, I think, sends the wrong message to unvaccinated people.”

Meanwhile, when states like Delaware and New Jersey provide real-world numbers, the case and vaccination data show a clear trend: “The vaccines are really working,” Morris said, “and the unvaccinated are vulnerable.”

Graphics editor John Duchneskie and staff writer Jonathan Lai contributed to this article.