The CDC’s color-coded vaccination map showed Pennsylvania had hit an impressive milestone this week: 95% of adults had gotten at least one dose of a COVID-19 vaccine. Hours later, Gov. Tom Wolf touted the figure at a news conference.

The staggering number would put Pennsylvania ahead of every other state and indicate that only about half a million adults in the state had yet to start vaccination.

But that number is wrong.

The true number is lower, yet almost impossible to calculate from publicly available data because official numbers are inaccurate in multiple ways. And the state’s top health official said she didn’t have an estimate.

The CDC is the only official source that compiles vaccine data for every jurisdiction nationwide, including counts of people who crossed state lines to get vaccinated. Its state-by-state map has become a trusted source of vaccination rates for many people, news outlets, and health officials. The Pennsylvania Department of Health regularly cites CDC numbers, including in daily news releases as recently as Friday, and many other states cite it on their online COVID-19 data dashboards.

Wolf was at a vaccine clinic in Reading when he gave Wednesday’s shout-out to the CDC’s numbers, calling them “pretty good” and erroneously attributing them to the Department of Health.

But acting Pennsylvania Health Secretary Alison Beam told The Inquirer this week that the CDC’s first-dose number should not be considered “a true metric” of where the state stands, and the data posted by Pennsylvania and Philadelphia are more accurate. Meanwhile, Philadelphia for months has been discouraging people from relying on CDC data after seeing errors in its numbers for the city: “We have and have had no confidence in their data,” spokesperson James Garrow said.

On Friday, a Pennsylvania Department of Health spokesperson clarified that it still encourages people to use the CDC vaccine tracker “for ease of use regarding state-to-state comparisons,” despite its inaccuracies.

Elizabeth Rementer, a spokesperson for the governor, said “all data sets have different limitations,” but “the fact is more people are getting vaccinated every day.

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

Why are the numbers wrong?

It’s not clear what the full issues are with the CDC data, what causes them, and whether they can or will be fixed.

The most well-known of the CDC’s data issues for Pennsylvania is that it overcounts first doses and slightly undercounts those fully vaccinated — and it has since July 9, when the state completed a data cleanup. The state provided those updated numbers to the CDC months ago, the secretary said, but the agency hasn’t adjusted its data. It also hasn’t adjusted for a separate data reconciliation that Pennsylvania conducted last month.

The number of first doses was overcounted by more than half a million. The number of fully vaccinated people was undercounted by about 64,000.

That 500,000 overcount means that any CDC rate for the state’s total population will be inflated by 4%.

A CDC spokesperson said Friday that “the vaccine administration process is complex,” but did not directly answer whether it was working to fix the overcounted Pennsylvania data. In September, a spokesperson had told The Inquirer that the agency was aware of the issue but had no timeline for rectifying it.

And the solution isn’t as simple as turning to the vaccine statistics published by Philadelphia and Pennsylvania — those are themselves a known undercount, because they don’t include vaccinations done out of state or by federal providers, which are counted in the CDC data.

Comprehensive, up-to-date data have been a critical tool for gauging the country’s progress in curbing the pandemic and informing policy decisions. The inaccuracy of the CDC’s Pennsylvania figures paints a misleading picture of the level of immunity the state has reached and how it differs across communities — at a time when public health experts say these numbers remain important now that most restrictions are lifted and people must make risk assessments for themselves and their families every day.

It’s relevant, too, as government, nonprofit, and medical workers address reluctance and access issues still keeping millions of people from getting shots. An inflated number could give the impression that the work of reaching every unvaccinated person is closer to being finished than it is or even exacerbate misinformation about the vaccine’s effectiveness, fueling conspiracies that vaccines don’t work if 95% of adults are immunized and the virus is still circulating.

What is the real number?

When looking at the entire population, the CDC says 80% of all Pennsylvanians, including those under 5 who aren’t yet eligible, have had at least one dose. If you adjust for the 500,000 overcount, that’s 76%.

The city and state numbers indicate about 66% of all Pennsylvanians have had at least one dose. Of course, those numbers are a known undercount.

So reality is likely somewhere between the two.

As for adults and the CDC’s 95% figure, the way Pennsylvania and Philadelphia provide age-related vaccination data makes it impossible to determine the state’s first-dose rate for people 18 and older. For example, the state’s age categories are broken down by five-year increments, meaning there’s no specific number for adults.

Pennsylvania has not taken its own numbers and combined them with Philadelphia, in part because their electronic reporting systems “are not directly compatible,” Department of Health spokesperson Barry Ciccocioppo said.

Philadelphia has been manually updating its dashboard with state data to account for Philadelphia residents who got vaccinated in other counties, Garrow said, and the state said it was also working with Philadelphia to reconcile data.

Meanwhile, Philadelphia continues providing its data to the CDC, but health officials “dissuade people from using it,” Garrow said. (Once, he said, the city sent data and a week later received a CDC report with different numbers.)

And Pennsylvania continues reporting, too, as state officials “continue to try to ask the CDC for the ability to refresh our data,” Beam said.

On Friday, a CDC spokesperson said: “Pennsylvania (and all other reporting entities) have the ability to update or delete their data as new information becomes available. These changes will be displayed in the CDC COVID Data Tracker when registered and reviewed.”

He did not respond to a follow-up question asking whether the agency had received Pennsylvania’s requests or had a timeline for reconciling the data.

“The CDC has been working on piloting with a few states and offering a refresh to that data,” Beam said. But, “Pennsylvania still has not been afforded that opportunity.”