The vaccines themselves are delicate and require a fair bit of focus over time. Consider Moderna’s instructions for preparing its doses: Thaw the vials for 2.5 hours in a refrigerator set between 36 and 46 degrees. Then rest them at room temperature for 15 minutes. Do not refreeze. Swirl gently between each withdrawal. Do not shake. Inspect each vial for particulate matter or discoloration.

And then there’s this: Once open, a vial is good for only six hours. As vaccines go, that’s not very long. Some flu vaccine keeps almost a month.

“This is very different, administering this vaccine. The process, it takes a whole lot longer than any mass vaccination event that I’ve been involved with,” said Suri, a member of the Loudoun Medical Reserve Corps who joined her first clinic Dec. 28, to vaccinate first responders.

Of the first two covid vaccines on the market, Moderna’s is considered more user-friendly. Pfizer-BioNTech’s shot must be stored in specialized freezers at 94 degrees below zero. Once out of deep freeze, it lasts just five days under refrigeration, compared with 30 days for Moderna’s.

One thing the shots have in common: They last a paltry six hours once the first dose is removed from a vial. That short shelf life raises the stakes for the largest vaccination effort in U.S. history by forcing clinicians to anticipate the exact number of doses they’ll need each day. If they don’t get it right, precious stores of vaccine may go to waste.

During one recent clinic over several hours, Suri estimated she gave “maybe 25” shots, many fewer than the number of flu shots she’s given during similar clinics over the years.

With covid, she said, “the vaccine itself slows things down.”

The slow rollout has frustrated people who at Thanksgiving imagined millions of vaccines in arms by Christmas. Promises that 20 million would be vaccinated by New Year’s fell well short: Just 2.8 million had the first of two required shots by the end of December, according to data from the Centers for Disease Control and Prevention.

Public health officials say many factors are at play, including a shortage of workers trained to administer shots, covid protocols that require physical distancing at clinics and vaccine allocation numbers from the federal government that fluctuate by the week.

And then there are the logistics of the first covid vaccines, which are complex and make hyper-vigilant practitioners wary of opening too many vials over the course of each day, for fear that anything unused will have to be tossed. Vaccine providers also report wasted or spoiled doses to public health authorities.

“If you get to the end of your clinic and every nurse has half a vial left, what are you going to do with that vaccine?” Suri said. “The clock is ticking. You don’t want to waste those doses.”

That impulse has led some health personnel to make dramatic decisions at the end of a day: calling non-front-line health workers or offering shots to whoever is at hand in, say, a grocery store, instead of scrambling to find the health workers and residents of nursing homes in the government’s first tier for injections.

“We jumped and ran and got the vaccine,” said Dr. Mark Hathaway, an OB-GYN in the District of Columbia who received the first dose of a Moderna vaccine on Dec. 26 along with his wife, a registered nurse specializing in nutrition. Both clinicians received vaccines faster than anticipated at a Unity Health Care clinic when there were extra doses because fewer front-line health care workers than expected showed up.

“Health care workers have been priority 1a, so our first attempt has always been our staff,” said Dr. Jessica Boyd, Unity Health Care’s chief medical officer. Since then, the community health center network has broadened its criteria for extra doses to include staff members or high-risk patients visiting a clinic, she said.

Health officials encourage using the doses to get as many Americans vaccinated as quickly as possible. Public health experts say the need to vaccinate people is especially urgent as a new and more contagious variant of the virus first detected in the United Kingdom is showing up in multiple states. Some states, including New York and California, have loosened their guidelines on who can get vaccinated after an outcry over health care providers throwing away doses that didn’t meet officials’ strict criteria.

The tiers “are simply recommendations, and they should never stand in the way of getting shots in arms instead of keeping vaccine in the freezer or wasting vaccine in the vial,” Health and Human Services Secretary Alex Azar said Jan. 6, referring to CDC guidelines saying health care workers and residents and staff of long-term care facilities should be first in line, then people at least 75 years old. The Trump administration this week also said it would make more shots available by releasing second doses and urged states to broaden rules to allow anyone 65 or older and any resident with a serious medical condition to get a shot.

Pfizer-BioNTech’s ultra-cold storage requirements have made it less ideal for local public health departments and rural areas.

Both of the available vaccines arrive in multidose vials — Pfizer-BioNTech’s contains about five doses, Moderna’s 10. Neither contains preservatives and they are viable for only six months frozen. By contrast, during the H1N1 pandemic roughly a decade ago, the swine flu vaccines lasted 18 weeks to 18 months, Sen. Chuck Grassley (R-Iowa) wrote in a May 2010 letter to then-HHS Secretary Kathleen Sebelius.

“We can’t get the vaccine out fast enough; we have people dying. But, at the same time, we have to get it right,” said Claire Hannan, executive director of the Association of Immunization Managers.

The added risk of losing doses due to quick expiration is another thing “causing angst,” Hannan said. “You can’t just draw it up and let it sit. It can’t just sit out like that.”

The Trump administration fell significantly short of its promise that 20 million Americans would be vaccinated by the end of December, partly the result of a disjointed and underfunded public health system that has received limited guidance from federal officials. As of Jan. 13, 29.4 million vaccine doses had been distributed nationwide but only 10.3 million administered, according to the CDC.

Federal officials have released sparse data about who is getting vaccinated, but state information has shown significant variation in vaccination rates depending on the facility. New York Gov. Andrew Cuomo on Jan. 4 said New York City’s public hospital system had used only 31% of its allocated vaccines, while private health systems NewYork-Presbyterian and Northwell Health had used 99% and 62%, respectively.

“When you target a priority group, it’s inefficient. When you open it up to a larger group, it’s efficient … but you’re not going to have enough supply,” Hannan said. “You still have the challenge of getting those health care workers vaccinated and no matter any way you slice it, you still have limited supply. You can’t please everyone.”

While Pfizer’s vaccine has largely been earmarked for large institutions like hospitals and nursing homes, Moderna’s has been more widely distributed to smaller sites like public health departments and clinics run by volunteers. State and local officials have begun or will soon vaccinate other priority populations, including police officers, teachers and other K-12 school employees, and seniors overall.

Unlike the covid vaccines, many flu vaccines come in prefilled syringes — each syringe’s cap is removed only when a shot is given, which speeds the process and eases some concerns about storage. However, relying on prefilled syringes during a pandemic has its own complications, according to Michael Watson, former president of Valera, a Moderna subsidiary: They take up more fridge space. They’re more expensive. And they can’t be used for frozen products, he said.

“For all these reasons, a vial was the best and only option,” he said.

In Ohio, Eric Zgodzinski, health commissioner for Toledo-Lucas County, said two-thirds of first responders the county surveyed said they would get the vaccine. Still, he said, his department has encountered situations in which a covid vaccine dose is left over in an open vial and officials have turned to a waiting list to find someone who can arrive within minutes to get a jab.

His department also has an internal running list of potential vaccine takers, including health department staffers, people in congregate care settings or those who had scheduled vaccination appointments for later on.

“We’re not going to open up a vial for one individual and figure out nine other people right away,” said Zgodzinski, whose department planned to distribute 2,200 doses of the Moderna vaccine the week of Jan. 4.

“If I have one dose left, who can I give it to?” he added. “A shot in the arm for anybody is better than it being wasted.”

San Francisco editor Arthur Allen and senior correspondent JoNel Aleccia contributed to this report.

Kaiser Health News (KHN) is a national health policy news service. It is an editorially independent program of the Henry J. Kaiser Family Foundation which is not affiliated with Kaiser Permanente.