A possible explanation is emerging for why federal data shows only about half of the vaccine supply delivered in the US has been administered.
The nation's vaccine distribution figures have baffled observers for weeks, with states claiming they need more vaccine when the data indicates they still have many doses on hand.
Health officials for President Joe Biden sought to explain on Wednesday, at least in part.
Speaking at a media briefing, Dr. Rochelle Walensky, director of the US Centers for Disease Control and Prevention, said not all vaccine that's been delivered to states is available for "inserting into people's arms."
White House Covid-19 Response Coordinator Jeff Zients took that explanation a step further.
"Some of what the states have right now is inventory to do the very, very important second shot," Zients said. "I think it's important that when you're looking at state's inventories that you recognize that some of that inventory is being held for the very important second shot."
Walensky, when asked by CNN Chief Medical Correspondent Dr. Sanjay Gupta on Wednesday if it is the right approach for states to hold onto Covid-19 vaccine that is intended for second doses, declined to directly answer the question.
An administration official on Wednesday told CNN states should not be holding back second doses and that is not the Biden administration's guidance.
The Pfizer and Moderna Covid-19 vaccines currently available for emergency use in the US require two doses. A federal dashboard tracks the nation's distribution of these vaccines. The data shows how many doses of vaccine have been delivered to each state, but it does not differentiate between first and second doses.
Consider Florida, where the federal data on Wednesday showed about 3.1 million doses had been delivered and about 1.6 million had been administered. That's roughly 50% of the doses going into arms.
On Monday, White House Press Secretary Jen Psaki used similar figures to suggest that Florida had a good deal of vaccine, after Florida governor Ron DeSantis claimed the state wasn't getting enough supply from the federal government and needed more.
"I will note, because we're data-first here, facts-first, they've only distributed about 50% of the vaccines they've been given in Florida," Psaki said. "So, clearly they have a good deal of the vaccine."
On Wednesday, DeSantis pushed back against those comments from the White House, explaining the federal data didn't account for vaccine earmarked for second doses.
"When the person at the White House says that Florida has all these doses, those are second doses," DeSantis said.
Other states also say part of their vaccine inventory is intended for second shots.
"When a first dose comes, you can just go ahead and give it to someone. When a second doses comes, it needs to be 21 days later for Pfizer or 28 days later for Moderna," Kristen Ehresmann, the director of Minnesota's Infectious Disease Epidemiology, Prevention and Control Division, said in an email to CNN. "So yes, we get this vaccine and then give it at the appropriate interval and it can look like we are 'sitting on doses' when that is not the case."
In New York, Governor Andrew Cuomo has started to express the state's vaccine distribution figures in terms of first and second doses, announcing Wednesday that 96% of the state's allocated first doses have been administered, excluding the federal long-term care facility vaccination program with CVS and Walgreens.
On Tuesday, Cuomo said his state was "basically out of vaccine," but that same day New York City Mayor Bill de Blasio, who had also complained about being short on vaccine, said the city did have doses intended for second shots in its inventory.
"I've got a hundred thousand second doses," de Blasio told MSNBC.
De Blasio went on to say the doses were "sitting on a shelf" and "can't be used for weeks." He said President Biden should order governments across the country to take second doses in their inventory and use them right now for first doses.
"Even a first dose gives folks about 50% protection," he said.
Cuomo said on Tuesday second doses aren't being distributed as first doses due to uncertainty over how quickly additional doses of vaccine can be made.
"The fear is, until you really know what the production schedule is, if you start using the second dose as the first dose, you have to have a dramatically increased supply otherwise you're going to leave people without a second dose when their appointment is due," said Cuomo.
It's unclear how many states have an inventory of second doses, or how many states may be handling distribution of second doses differently. The White House and the Health and Human Services Department did not immediately respond to CNN's inquiries for additional details.
In Maryland, the state is not holding onto any reserve doses in its warehouses aside from doses to be administered that week, according to Charles Gischlar of the state's department of health. Gischalar says Maryland has requested the federal government automatically distribute second doses to providers who were provided with first doses.