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With another pandemic surge possibly on the way, vaccination for the coronavirus in the United States has all but ground to a halt, with initial doses and boosters plummeting to the lowest levels since the program began in late December 2020.

On Wednesday, the seven-day average of vaccinations fell to fewer than 182,000 per day, according to data compiled by The Washington Post. That is lower than at any time since the first days of the program.

The daily total has been in free fall for the past six weeks. On Feb. 10, the nation was averaging more than 692,000 shots a day. Booster shots have been more common than first or second doses since October, and the low rates have long caused concern among some experts.

Now, with authorities bracing for a possible increase in covid-19 cases caused by the BA.2 subvariant, 65.4 percent of Americans are fully vaccinated and just 44 percent have received a booster shot. That is substantially less than the totals in many Western European nations - which nevertheless have seen a sharp rise in cases in recent weeks and months.

Federal health officials are now considering authorizing fourth shots for people 65 and older. But the nation's booster campaign, which was initially plagued by conflicting guidance and disagreement among advisers and scientists, has faltered: People who were willing to roll up their sleeves for first and second doses are seemingly less inclined to go for a third.

"This is an unforgivable liability that we did not get people boosted at a much higher level," said Eric Topol, director of the Scripps Research Translational Institute in San Diego.

A growing body of research has borne out the need for booster shots. The World Health Organization, which at first questioned the push for boosters in a moment when other countries lacked vaccination coverage, has since reversed course and offered its endorsement.

In a recent interview, Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, called the data "strong and highly convincing about the benefit of the booster." While he said "nothing surprises me anymore," he expressed disappointment that fewer than half of vaccinated people are boosted - and said the United States needs to do better.

"If boosting was an iffy situation, that it doesn't help, then you could see, 'Well, what's the difference?'" Fauci said. "But it is a difference, because if you look at the data with boosting, it absolutely brings up the protection to a very high level."

But a majority of the vaccinated public has not been convinced.

Kyle Elston was so eager to get a coronavirus vaccination that he leaped ahead of his age group, listing himself as obese.

When authorities recommended booster doses months later, scheduling an appointment topped his to-do list - until he contracted covid. Thrown off by unclear guidance on whether an extra shot was needed post-infection, the 45-year-old New York resident hasn't gotten one. He's not sure he ever will.

"The first two shots, it made total sense. It was a no-brainer," Elston said. "I don't know that it's as clear anymore with a booster."

The Centers for Disease Control and Prevention says in an FAQ to health-care professionals that vaccinated people who have recovered from prior infections can receive a booster dose. But the guidance is difficult to find and recommendations on the timing of the shot is unclear.

A CDC spokeswoman did not respond to a request for comment. Representatives from the White House and the Department of Health and Human Services did not respond to an email asking about the declining number of vaccinations.

A spokeswoman for CVS Pharmacy, which has shots available at 9,000 locations in the United States, said in an email that its pharmacies "adjust our orders frequently to account for COVID-19 case trends. The government has also transitioned to allocating vaccine to pharmacies every two weeks, rather than every week, as was done earlier in the pandemic, and their allotments have declined based on recent trends as well."

A spokeswoman for Walgreens, which has administered 60 million doses of vaccine, said in an email that the company remains "committed to providing access to these services."

Polling by the Kaiser Family Foundation shows the group that refuses even a first vaccination has remained at 12 to 16 percent since December 2020, a remarkably consistent segment of the population, according to Liz Hamel, the organization's vice president and director for public opinion and survey research.

"The job gets harder and harder over time," she said. "We see nothing in terms of messages or mandates that will move that 12-16 percent."

Among people who were vaccinated but not boosted, 35 percent told KFF pollsters in February that they would get a booster as soon as possible. But 24 percent said they would do so only if required, 23 percent said they definitely would not and 16 percent said they would wait and see.

Democrats were more likely than Republicans to report they had received a booster or planned to get one soon. Black and Hispanic adults lagged their White counterparts in getting boosted, though it was not clear from the poll whether that trend was driven by vaccine hesitancy or barriers to access.

At the University of Pennsylvania Health System, a series of vaccination efforts were held at locations in the Philadelphia area in February through May 2021, with a focus on reaching the city's Black residents. But demand slumped. There was a slight uptick brought about by the emergence of the delta variant and the recommendation that adults get boosters. That lasted through mid-January, when "it dropped off a cliff again," said Patrick J. Brennan, chief medical officer of the University of Pennsylvania Health System.

"Without that sense of urgency from a highly transmissible variant, people just stopped seeking it," Brennan said.

Because of staffing issues, he said, the medical center has not been able to maintain the same level of community outreach as before. It has not, for instance, called and texted people who are eligible for boosters. And its on-site clinics will close March 31, though people will be able to make vaccination appointments at some of the hospital's medical practices.

"Part of what enabled our ability to be outside our four walls and in the community was the capacity that we had with staff who were willing to volunteer or that we were able to deploy," he said. "We just don't have as many staff that are as available."

Overall, one of the most commonly named reasons for skipping the extra shot was the belief it isn't needed, according to a KFF survey from January. That explanation was given by 13 percent of non-boosted adults. Another 12 percent said they were not eligible for a booster. Others had doubts about the effectiveness of boosters - 9 percent cited the fact that vaccinated people were still getting sick.

According to the KFF February vaccine monitor, the booster rate was 68 percent among adults 65 and older but 34 percent among those 18 to 29 years old.

Travis Pemberton, 30, of Michigan said he saw getting his first and second shots as "the responsible thing to do." But while he thinks people who are older or at increased risk should get a booster, he's skeptical that it's necessary for him, as a healthy, younger person.

"A couple years down the road, I wouldn't be opposed to getting a booster shot," he said. "Right now, it's like, I just got the vaccine - do I seriously need a booster?"

The failure to persuade the vaccinated to get boosted is "the most pitiful aspect of what we've done in the vaccine campaign," Topol said, adding that "these are not anti-vaxxers."

With fourth shots already on the horizon for some, he said that failure raised concerns about whether Americans would be willing to roll up their sleeves if needed again in the future.

"How can we do justice to the fourth shot when we have totally botched the third?" he asked.

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The Washington Post's Akilah Johnson and Emily Guskin contributed to this report.