WASHINGTON — When Health and Human Services Secretary Alex Azar announced this week that the federal government would begin releasing coronavirus vaccine doses held in reserve for second shots, no such reserve existed, according to state and federal officials briefed on distribution plans. The Trump administration had already begun shipping out what was available beginning at the end of December, taking second doses directly off the manufacturing line.

Now, health officials across the country who had anticipated their extremely limited vaccine supply as much as doubling beginning next week are confronting the reality that their allocations will not immediately increase, dashing hopes of dramatically expanding eligibility for millions of elderly people and those with high-risk medical conditions. Health officials in some cities and states were informed in recent days about the reality of the situation, while others are still in the dark.

Because both of the vaccines authorized for emergency use in the United States are two-dose regimens, the Trump administration’s initial policy was to hold back second doses to protect against the possibility of manufacturing disruptions. But that approach shifted in recent weeks, according to the officials, who spoke on the condition of anonymity because they were not authorized to discuss the matter. The result is that next week’s allocations will remain flat.

These officials were told that Operation Warp Speed, the Trump administration’s initiative to speed the development of vaccines and therapeutics, stopped stockpiling second doses of the Pfizer-BioNTech vaccine at the end of last year, instead taking second doses directly off the manufacturing line. The last shots held in reserve of Moderna’s supply, meanwhile, began shipping out over the weekend.

The shift, in both cases, had to do with increased confidence in the supply chain, so that Operation Warp Speed leaders were confident they could reliably anticipate the availability of doses for booster shots — required three weeks later in the case of the Pfizer-BioNTech product and four weeks later under Moderna’s protocol.

But it also meant there was no stockpile of second doses waiting to be shipped, as Trump administration officials suggested this week. Azar, at a Tuesday briefing, said, “Because we now have a consistent pace of production, we can now ship all of the doses that had been held in physical reserve.” He explained the decision as part of the “next phase” of the nation’s vaccination campaign.

» READ MORE: Biden taps former FDA chief David Kessler to lead vaccine science

The lack of reserve is not expected to interfere with people getting their second shots. States are still getting a regular supply of vaccine, if smaller than they had anticipated, but officials say they are angry and bewildered by the shifting directions.

HHS spokesman Michael Pratt confirmed in an email that the final reserve of second doses had been released to states for order over the weekend but did not address Azar’s comments this week, saying only, “Operation Warp Speed has been monitoring manufacturing closely, and always intended to transition from holding second doses in reserve as manufacturing stabilizes and we gained confidence in the ability for a consistent flow of vaccines.”

He also said states have ordered only about 75 percent of what is available to them.

Azar’s comments followed a Jan. 8 announcement by President-elect Joe Biden’s transition team that his administration would move to release all available doses, rather than holding half in reserve for booster shots. Biden’s advisers said the move would be a way to accelerate distribution of the vaccine, which is in short supply across the country.

When Azar embraced the change four days later — after initially saying it was shortsighted and potentially unethical to risk the availability of a booster shot -he did not say the original policy had already been phased out, or that the stockpile had been exhausted. Signaling to states that they would soon see expanded supply, he also urged them to begin vaccinating adults 65 and older and those under 64 with a high-risk medical condition. Officials in some states embraced that directive, while others said suddenly putting hundreds of thousands of additional people at the front of the line would overwhelm their capacity.

In subsequent conversations with state and local authorities, federal officials sought to temper those instructions, said people who participated in the conversations. Gustave F. Perna, chief operating officer of Operation Warp Speed, spoke directly to officials in at least two of the jurisdictions receiving vaccine supply, explaining that allocations would not increase and that they did not have to broaden eligibility as they had previously been told, according to a health official who was not authorized to discuss the matter.

» READ MORE: Hundreds of thousands of COVID-19 vaccines haven’t been used in Pa. and N.J. Here’s why, and what’s next.

The health director in Oregon, Patrick M. Allen, was so disturbed by the realization that he wrote a letter Thursday to Azar demanding an explanation.

“Earlier today, we became concerned when we discovered there were no additional doses available for allocation,” he said in the letter, which was reviewed by The Washington Post.

On a call with Perna earlier the same day, Allen wrote, the four-star Army general “informed us there is no reserve of doses, and we are already receiving the full allocation of vaccines.”

“If true, this is extremely disturbing, and puts our plans to expand eligibility at grave risk,” Allen added. “Those plans were made on the basis of reliance on your statement about “releasing the entire supply” you have in reserve. If this information is accurate, we will be unable to begin vaccinating our vulnerable seniors on Jan. 23, as planned.”

The revised instructions led additional jurisdictions to hold off on broadening their priority groups. One state health official noted that the updated guidance announced Tuesday as to eligibility did not appear on the website of the Centers for Disease Control and Prevention, even though it was stated as federal policy by Azar and by Robert R. Redfield, the CDC director, in their remarks earlier this week. Under the original recommendations, adults 65 and older and frontline essential workers were to comprise the second priority group, known as phase 1b, in line after medical workers and residents and staff of long-term care facilities.

But the issue of supply was most troubling to state health officials.

“States were shocked and surprised that they did not see an increase in their allocations, and when they asked for explanations, some of them were told there was not a large stockpile of second doses to draw from,”said an official working with numerous states on vaccination planning who spoke on the condition of anonymity to recount sensitive conversations. “They thought they were getting more doses and they planned for more doses and opened up to 65 and up, thinking they were getting more.”

In an email that reached some state officials on Friday morning, Christopher Sharpsten, an Operation Warp Speed director, called it a “false rumor” that “the federal government was holding back vaccine doses in warehouses to guarantee a second/booster dose.”

But it had been Azar who said Tuesday hat one of the changes being made in the government’s “next phase” of vaccination was that “we are releasing the entire supply we have for order by states, rather than holding second doses in physical reserve.”

There was additional confusion. Another change Azar announced this week — making allocation of doses dependent on how quickly states administer them — would not take effect for two weeks, he said. But Connecticut Gov. Ned Lamont, D, on Thursday tweeted that federal officials had notified the state that it would receive an additional 50,000 doses next week “as a reward for being among the fastest states” to get shots into arms. West Virginia, meanwhile, which is moving at the fastest clip based on CDC data, did not get any additional doses, said Holli Nelson, a spokeswoman for the state’s National Guard.

In a sign that the incentive structure may not be long-lived, a senior Biden transition official, speaking on the condition of anonymity to address ongoing deliberations, said this week the team did not look kindly on a system that “punishes states.”