The U.S. COVID-19 vaccine rollout moved slower than expected this month, stoking fears that the federal government was mishandling the effort and that people would have to wait months longer than expected to get their shot.
Federal officials defended the program, insisting that bringing the vaccine to the American people in less than a year was still a technical feat and that the program remained on track.
Still, they acknowledged the effort was lagging behind expectations, and vaccine experts and public health officials warned the bigger test will come next year when inventory finally expands and the broader public raises their hands for a shot.
"It's really difficult to administer every dose when you are prioritizing it and trying to avoid waste," said Claire Hannan, executive director of the Association of Immunization Managers.
"But when we get into a position of mass clinics and everyone has access, we'll be much more efficient in getting it out," she said.
President Donald Trump had initially pledged 300 million doses by January 2021 when announcing Operation Warp Speed, then later this fall dropped the estimate to 100 million. After Pfizer adjusted its production estimates, Health Secretary Alex Azar promised 40 million doses on hand and 20 million vaccinations by the end of the year.
Instead, the administration was on track to ship those 20 million doses by the first week of January -- enough for first doses in the two-dose vaccine -- with only 2.6 million vaccinations recorded by the federal government.
Trump blamed the states for not moving fast enough.
"The Federal Government has distributed the vaccines to the states. Now it is up to the states to administer. Get moving!" he tweeted Wednesday.
Just hours later, Trump's top scientific adviser on the vaccine program, Moncef Slaoui, acknowledged "the number is lower than what we hoped for."
"There is a learning curve in the system," Slaoui told reporters.
Vaccine experts and public health officials said they aren't ready to sound the alarms just yet, but they are citing numerous smaller logistical challenges that have complicated the rollout: a vaccine that has specific handling requirements, and hospitals that must stagger injections for front-line hospital employees based on the latest shipment numbers.
Holidays and snowstorms haven't helped, and a federally run partnership with major pharmacies to deliver vaccines in nursing homes only just got started. Also, states participating in that program were required to hold some doses in reserve.
"Receiving, preparing and administering vaccines takes time," said Kris Ehresmann, director of the infectious disease division at the Minnesota Department of Health.
New Jersey Health Commissioner Judy Persichilli suggested that Pfizer vaccinations are going at a fast pace -- almost 60% of the doses administered so far -- but Moderna doses have been stalled a bit because they were "delivered right before Christmas."
"There were no vaccinations on the 25th, and on the other days, people didn't want to be sick -- they were concerned that we're not going to feel well for the holidays, so it's ticked off again," said Persichilli.
Likewise, the Pennsylvania Department of Health cited the holidays and a major snowstorm paired with the challenge of vaccinating a hospital staff already stretched thin. The Colorado Department of Public Health said hospitals were trying to stagger vaccinations among employees in case there are temporary reactions that prompt staff to call in sick.
"I really do expect next week, when the holidays are over, for those numbers to rapidly jump as jurisdictions move ahead quickly to protect their health care personnel, and also long-term care facility residents," said Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention.
Blaire Bryant, associate legislative director for health at the National Association of Counties, agreed that the slower-than-expected rollout isn't a problem yet. But counties are concerned about whether there's enough money to see it through a nationwide rollout in spring, she said.
The federal government in recent months has sent $340 million to the states, but that money has been slow to trickle down as cash-strapped states sort through competing priorities, creating what Bryant called a "barrier" that could be addressed with direct, flexible cash grants. On Sunday, Trump signed a $900 billion COVID relief bill that included more than $8 billion for vaccine distribution.
Bryant said many overwhelmed local communities also could use help to support public messaging on the vaccine, as well as more details on what to expect in coming weeks. Initial allotment was based on each state's adult population. It's not known whether federal officials could change that formula to account for outbreaks, or whether a community could get to pick the vaccine of their choice.
"It's a slow rollout right now, and we'll wait and see as we move toward that mass vaccination stage. ... If it doesn't meet the needs on the ground, we'll cross that bridge when we get there," Bryant said.
Hannan, from the Association of Immunization Managers, agreed that expanding the vaccine rollout behind health care personnel was her biggest concern. By the time hospitals and nursing homes are covered and people over the age of 65 and essential workers are invited to get a shot, there will be less concern about fair allocation. But the government will need to have enrolled enough providers to roll it out nationwide next spring.
That means enlisting primary care physicians, local pharmacies and others to jump on board with federal requirements that show they can store, handle and administer the vaccines properly.
"Whatever needs to be done will be done," Hannan said. "But there will be hitches."
Maryland Gov. Larry Hogan told "CBS This Morning" that overall the program is behind in part because the federal government overpromised on deliveries earlier this fall and production levels changed. But the logistical challenges are considerable, too.
"It's not just sticking needles in arms; there's a lot of moving parts. I think nobody is quite performing at top capacity and we've all got to work together to ramp it up," he said.
ABC News' Sophie Tatum contributed to this report.