The Centers for Disease Control and Prevention, which millions of Americans rely on for up-to-date information on vaccination rates in their communities, recently acknowledged that its data might overestimate the number of people who have received first doses while underestimating the number who have received booster shots.

The acknowledgment was easy to miss, tucked into footnotes at the bottom of the vaccination tracking page on the CDC website. It said that, in light of the possible error, the agency would cap its estimates of vaccination rates at 95%. Previously, it had capped its estimates at 99.9% and, for example, showed a 99.9% national vaccination rate for people 65 and older, which experts said was clearly inaccurate.

The CDC’s data on vaccination rates is still considered to be reliable, especially with regard to the number of fully vaccinated Americans, experts say.

The main reason for the discrepancies is that state and county data, which the CDC relies on to compile its statistics, does not always properly link the record of people’s booster shots to the records of their initial vaccinations. When the two are not connected, the booster is recorded as if it were a first dose given to a previously unvaccinated person.

This can happen when people go to a different location for a booster shot than they did for their original series of injections. That often occurs when people move, or the place they received their first doses doesn’t exist anymore, as is the case with many government-sponsored mass vaccination sites that closed after a few months. Sometimes a different location for a booster is chosen simply because it’s more convenient.

Data reported to the CDC is stripped of personal information, which makes it difficult to spot and correct these sorts of errors.

“Even with the high-quality data CDC receives from jurisdictions and federal entities, there are limits to how CDC can analyze those data,” the agency said in one of its footnotes. The note added that people receiving boosters at a different location was “just one example of how CDC’s data may overestimate first doses and underestimate booster doses.”

This article originally appeared in The New York Times.

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