Pfizer said it had millions of doses of vaccine sitting in a warehouse because the federal government hadn't told it where to send them

Pfizer released a statement Thursday to deny claims it was having issues producing its COVID-19 vaccine.

It came after officials in more than a dozen states complained that they were receiving fewer doses than promised and the US secretary of health and human services mentioned "manufacturing challenges."

In its statement, Pfizer said it was "not having any production issues" and in fact had millions of doses sitting in a warehouse, awaiting government orders on where to ship them.

Two anonymous Trump administration officials told the Associated Press the doses were in storage on purpose, held back to ensure that people who had their first shot of the two-dose vaccine could get the second one.

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Pfizer said Thursday that millions of COVID-19 vaccines were sitting in storage because the US government hadn't given it directions on where to send them yet.

The statement was released a day after Health and Human Services Secretary Alex Azar suggested at a press conference that the drug company was having "manufacturing challenges," The Hill reported.

Governors and health officials in more than a dozen states have also complained about getting fewer doses than they expected, according to the Associated Press.

Illinois, Washington, California, Georgia, Hawaii, and Nevada are getting about half the doses they expected, according to the AP and NBC News. Sandra Lindsay, a nurse at Long Island Jewish Medical Center, being inoculated with the Pfizer COVID-19 vaccine by Dr. Michelle Chester at Long Island Jewish Medical Center on Monday. Mark Lennihan/AFP/Getty Image

Pfizer said in the Thursday statement that the company was "not having any production issues with our COVID-19 vaccine" and that "no shipments containing the vaccine are on hold or delayed."

"This week we successfully shipped all 2.9 million doses that we were asked to ship by the US Government to the locations specified by them," the statement added.

"We have millions more doses sitting in our warehouse, but, as of now, we have not received any shipment instructions for additional doses."

Two senior Trump administration officials who spoke anonymously with the Associated Press on Thursday said the doses were being held at Pfizer's Michigan manufacturing plant on purpose.

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It had always been the plan to distribute only 2.9 million of the 6.4 million doses produced in a first wave, the officials said.

Another 2.9 million are in reserve to make sure people who got the first part of the two-shot vaccine would be able to get the necessary second dose, they said.

Another 500,000, per the officials, were being held to the side in case of any unforeseen problems.

The unnamed officials blamed the complaints from state officials on a misunderstanding and said the full allocation would arrive.

They said the government had changed the delivery schedule of the vaccine so states weren't getting all of their allotment at once, at the request of governors, who hoped spaced-out deliveries would be more manageable.

"They will get their weekly allocation, it just won't come to them on one day," one official told the AP.