After a month of dramatic twists and turns more suited to a soap opera than a staid federal health bureaucracy, the White House this week got much of what it hoped for: access to Pfizer-BioNTech coronavirus booster shots for most adults at least six months after completing the standard two-dose regimen.

But the messy process leading to this moment sparked massive confusion about the booster program, critics say, and may distract from efforts to get first shots to the unvaccinated.

In a statement issued just before midnight Thursday, the Centers for Disease Control and Prevention recommended extra shots for everyone 65 and older as well as those 50 and older with underlying medical conditions such as diabetes. It also said extra shots may be considered for younger people with medical conditions and for those whose jobs or living situations are putting them at risk for serious cases of covid-19, the illness caused by the coronavirus.

But the reality of who will have access to third shots, experts said, is much starker: Most adults who want a Pfizer-BioNTech booster will be able to get one simply by saying they are in one of the designated groups. Such “self-attestation” does not require a doctor’s note or other verification.

“The door just got opened,” said Paul A. Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, who says boosters won’t curb the pandemic and should be used in a more targeted way, partly because of a lack of safety data for younger people. “The administration just got what it wanted: three-dose fever.”

Peter Hotez, professor of pediatrics and molecular virology at Baylor College of Medicine, agreed the CDC recommendations “are pretty leaky,” adding that most people could argue, for example, that their jobs put them at risk for covid-19, unless “you are a writer of novels in the backwoods of Maine.”

But he said he supports much wider use of boosters, in part to counter the threat of long covid syndrome, which can cause muscle pain and neurological problems for months or years, and to allay fears of front-line workers in occupations such as health care and teaching.

As a practical matter, many consumers who want an extra dose have already been able to get a shot by asserting they are immunocompromised, a group eligible since last month. Unlike earlier in the year, the nation is awash in vaccine. Of the more than 260 million doses of Pfizer vaccine that have been delivered, about 37 million doses have not been administered, according to the CDC. About 70 percent of vaccine doses are being given at pharmacies, which won’t require additional documentation for boosters, the CDC said.

“In line with CDC guidance, we are relying on self-attestation from customers with regards to their qualifying conditions as outlined in the agency’s recommendation and guidance,” CVS Health spokesman Joseph Goode said in an email Friday.

“It should be easy,” said one state immunization official who spoke on the condition of anonymity to speak candidly. “There won’t be heavy screening. No one has time or resources for that.”

But the number of booster-seekers is expected to rise sharply following the issuance of the new recommendations, weeks of public debate about the waning of vaccine efficacy and concerns about breakthrough infections among the vaccinated.

President Biden on Friday urged front-line workers, Americans older than 65 and people with underlying health conditions who got the Pfizer-BioNTech vaccine to get a third shot. He said 20 million people who received the vaccine are eligible to receive the booster and another 40 million will be soon.

“Six months after you receive the second shot, you’re eligible,” Biden said.

Jason Goldman, a Florida physician, said he has been deluged by patients seeking boosters.

“I have had nonstop questions about boosters all day,” said Goldman, a liaison member to the CDC advisory panel on immunization. He said some are confused by the process and “really have lost faith in the CDC because their perception is no one can make up their mind.”

The new booster recommendations were spelled out by CDC Director Rochelle Walensky, who accepted most of the recommendations from her panel of outside experts, the Advisory Committee on Immunization Practices.

But on the key issue of whether adults 18 and older should be eligible for a booster because their work or living situation places them at risk, she overruled her advisers, who in a vote narrowly opposed eligibility for that group. Walensky instead aligned herself with the Food and Drug Administration’s stance, which was to authorize boosters to people in higher-risk occupational and living situations.

The issuance of the CDC recommendations Thursday gave Biden, who had said in mid-August that boosters would be available for all adults, a win — one that does not go as far as initially sought but gets close.

Some observers said the process, with its numerous disagreements and reversals, was already igniting questions in the minds of the public.

“It’s just really, really, really confused, more than I think anyone could have expected, even with changing science and different vaccines and all the complexities that go into policymaking,” said Jason L. Schwartz, associate professor of health policy and the history of medicine at the Yale School of Public Health. “It’s a really bad place to be in at this really critical moment in the pandemic.”

Immunization advocates also worry some people will be more adept at taking advantage of what is essentially an honor system for boosters. That will widen inequities in the vaccination program, they fear.

At a White House covid-19 briefing Friday, Surgeon General Vivek H. Murthy said it is “extremely important” to the administration that people have clarity on the recommendations. He said officials have been working for the last month in planning the booster rollout with state and local health officials.

“We’ll continue to get information out through a variety of means to make sure people know how to put these recommendations into effect,” Murthy said.

The Biden administration had been debating the need for boosters for months, but the issue moved front and center Aug. 18, when top health officials announced a plan to begin offering extra shots for all three coronavirus vaccines this week in a bid to counter waning immunity and combat the delta variant of the virus. The officials said the plan would depend on sign-offs from the FDA and CDC.

The month that followed was marked by a remarkable amount of disagreement both inside the health agencies and between administration officials and their scientific advisers. One of the biggest issues exposed a philosophical divide: whether the United States should give vaccinated citizens boosters while many countries have little access to the shots. Another point of contention was whether sufficient data existed to make sweeping decisions.

As circumstances rapidly changed, goals shifted. Early on, it became clear the FDA would not have sufficient time and data to quickly authorize boosters for Moderna and Johnson & Johnson. Officials now are hoping they can act on those brands in the next few weeks.

In addition, officials initially said people would need to wait at least eight months after the second dose of the standard two-dose regimen for the mRNA vaccines before getting a booster. That timeline was based on it being roughly eight months since the first groups of Americans, including nursing home residents and health workers, had received vaccines last December and January, said a federal official who spoke on the condition of anonymity to discuss internal discussions. But the FDA decided to authorize the Pfizer-BioNTech booster for six months out because the companies had the most data for that time frame, the official said.

Two big surprises shook the march to boosters. In a highly unusual move, two senior vaccine officials at the FDA demonstrated their disapproval for the booster push by signing on to a medical journal article questioning the scientific and ethical rationale for a booster. In essence, they embraced the World Health Organization’s viewpoint on the issue.

At the CDC, the agency’s advisory committee on Thursday voted down, in a close vote, booster eligibility for people placed at risk because of their jobs or living situations, dialing back on one of the main parts of the FDA’s earlier authorization.

That left Walensky on Thursday night with a difficult choice: Accept her experts’ advice and split with the FDA, leading to conflicting messages from two of the government’s health agencies, or overrule the advisers and risk their wrath. She chose the latter course, saying at a White House briefing Friday that “it was a scientific close call” but she supported giving access to people in at-risk jobs and living situations.

“It was a decision about providing rather than withholding access,” she said.

A federal official who spoke on the condition of anonymity to discuss internal discussions said Walensky strongly favored the broader FDA approach but was uneasy about overruling her advisers.

“With the support of multiple government partners, she made her decision to override” the advisory committee, the official said.

Some members of the CDC advisory group are upset, but the consensus is that the decision was Walensky’s prerogative as director.

“It’s complicated and there is no right answer as seen by the split votes, and they stand by the open, public, honest and science-based process,” said an advisory panel member who spoke on the condition of anonymity to speak frankly. But the person added: “Also, it would be nice to get out the word that getting more people to get the primary series is the most important thing to do.”

An administration official who spoke on the condition of anonymity to discuss private conversations conceded that the booster process was messy. But the official said the administration push was needed to make the shots available now rather than later.

Analysts watching the process predicted the debate over boosters is not over.

“We think the lack of congruency among decision-makers at both the CDC and FDA highlights how challenging it is to make data-driven decisions in this environment, with fairly limited data but plenty of political pressure,” analysts at Baird Equity Research said in a note to investors.