Health officials are encouraging all American adults to get booster shots for additional protection against omicron—but because of data collection and reporting faults, "[t]he truth is, we have no idea" how many Americans have received their additional doses, according to state officials.

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CDC revises its vaccination data

Earlier this month, CDC acknowledged that its data likely overestimates the number of people who have received their first dose of a Covid-19 vaccine and underestimates those who have received booster doses, the New York Times reports.

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Specifically, the agency said that in light of a potential error, it had would cap the estimated vaccination rates for individuals over the age of 65 at 95%, down from 99.9%—where it had been capped for several weeks—without changing its raw shot totals, Bloomberg reports.

According to Axios, the revision confirmed what some officials already suspected—the government counted many shots as first doses that were actually second doses or booster shots—although experts said the totals on how many people are fully vaccinated are still reliable. 

For instance, CDC data estimates that around 72.5% of the U.S. population (roughly 240 million people) have received at least one dose, Bloomberg reports. However, the agency currently estimates that just 61.3% of the U.S. population, or roughly 203 million people, are fully vaccinated.

According to Axios, state and local officials say it's improbable that the entire 11 percentage-point difference (roughly 37 million people) who got one shot didn't complete their inoculations. And in three states in particular—Illinois, Pennsylvania, and West Virginia—officials said they found enough overcounting of initial doses to suggests that "millions" of unvaccinated people have erroneously been assumed to have received at least one dose, Axios reports.

Therefore, they say the government has regularly and incorrectly counted booster shots and second doses as first doses, meaning both the fully vaccinated and completely unvaccinated are both undercounted.

However, the extent of the miscount is currently unknown.

Why are there discrepancies in CDC's data?

According to the Times, the state and county data that CDC depends on to compile its statistics sometimes do not properly link an individual's booster shot record to the records of their initial vaccination. As a result, a booster dose can be incorrectly recorded as an initial dose given to a previously unvaccinated person.

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Typically, this occurs when an individual goes to a new location for their booster shot, such as when people move, or if the site where they received an initial dose is no longer active, such as a government-sponsored mass vaccination site.

However, CDC acknowledged that receiving boosters at a different location was "just one example of how CDC's data may overestimate first doses and underestimate booster doses." Notably, reporting methodology varies by state, the Times reports. And data reported to CDC is anonymized, which makes it difficult to spot and correct 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.

Still, experts said CDC's data on vaccination rates are still reliable—especially for the total number of fully vaccinated Americans, the Times reports.

How many Americans have received booster shots?

While several states, including Minnesota, Colorado, New Jersey, North Carolina, and Maine, have asked CDC for data revisions, others have said they are confident their numbers are accurate, Bloomberg reports.

But with discrepancies in vaccine reporting likely in the millions, James Garrow, a spokesperson for the Philadelphia Department of Public Health said, "We don't have any faith in the numbers on the CDC website, and we never refer to them."

"Where it has really made it difficult for us is targeting our booster messaging," he added.

"Given the complex nature of vaccine administration and data reporting in the United States, CDC has been actively working with partners at state and local levels to enhance the quality of vaccine data," the agency said. 

But regarding the number of Americans who have received booster shots? "The truth is, we have no idea," said Clay Marsh, West Virginia's Covid-19 czar. (Wingrove, Bloomberg, 12/18; Astor, New York Times, 12/11; Dam, Axios, 12/18)