I found a discrepancy in CDC vaccine stats. Here's what happened.

Last week, my husband was reading a New York magazine article about kids and COVID-19. "Huh," he said, "Pennsylvania is one of three states where 99 percent of people over 65 have been vaccinated."

I replied that his reading comprehension was bad, because that couldn't possibly be true. But sure enough, the story said Hawaii, Vermont, and Pennsylvania have all topped 99 percent vaccination among seniors. I could buy it for the first two, both blue states where vaccine hesitancy might be quite low. But purple Pennsylvania? No way. I decided to investigate.

The New York story cited The New York Times' vaccine statistics page, which also said 99 percent. The Times cited Centers for Disease Control (CDC) data, which linked to Pennsylvania's state vaccine dashboard, and there I found something quite different. Some counties hit that 99 percent vaccination rate for some five-year age spans above 65. But the statewide average for those senior age brackets was substantially lower, around 80 percent. (There was also a similar but smaller disparity in CDC reporting for at least one other state: Wisconsin.)

I reached out to the CDC for an explanation. The Pennsylvania dashboard, a representative of the agency's public affairs office told me, has the more accurate information because the state "identified and removed some duplicate records in their system." Though the "CDC is working with PA to also identify those records on the CDC COVID Data Tracker and make those updates there," at present, the state numbers are more reliable. Pennsylvania seniors' vaccination rate is around 80 percent, as the state reported, not 99 percent.

Another contributing factor the representative mentioned: It was not until mid-July, more than half a year into this vaccination campaign, that the CDC piloted a project "to allow for updates to vaccination records submitted to CDC." That ability "will be implemented more widely over the next few months," so that various government and health-care organizations "will eventually be able to update previously submitted records, or add new [demographic] information."

After that happens, CDC data will be more accurate than it is now. But until those updates are done, the CDC vaccine tracker includes unknown errors (like the duplicates from Pennsylvania) accumulated in the seven months between mid-December 2020 and about two weeks ago, during which time the CDC had no formal process in place to fix mistakes in the vaccine data.

There will always be some disparity between CDC and state numbers, the representative told me, because the CDC may source additional statistics from "federal entities" and may have differences of "reporting schedules, data cleaning, and lag time." But right now, while that newly piloted update program takes months to ramp up, the differences could be larger, as they were with Pennsylvania.

The CDC's failure to include an update option in its vaccine tracker from the beginning means its data is overdue for a cleaning. And because it's the CDC, that uncorrected data has spread — to New York magazine, for example, via The New York Times, creating the false impression that essentially all Pennsylvania seniors have vaccine protection against COVID-19. They don't.

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