Top federal health officials have warned the White House that the Biden administration’s plan to begin offering booster shots to most Americans later this month may have to be limited initially, with third shots made available only to people who had received the Pfizer-BioNTech coronavirus vaccine, according to people familiar with the matter.

Janet Woodcock, acting commissioner of the Food and Drug Administration, and Rochelle Walensky, director of the Centers for Disease Control and Prevention, told White House coronavirus coordinator Jeff Zients on Thursday that their agencies may not be able to approve a more expansive coronavirus booster plan that they, along with other top doctors across the administration, endorsed last month.

Woodcock and Walensky told Zients that, by the end of this month, they may be able to approve and recommend booster shots only for people who received the Pfizer-BioNTech vaccine. Some officials said Friday that reviews of the other vaccines’ boosters could take an additional few weeks, though they cautioned it depends on the data.

The people familiar with the discussion spoke on the condition of anonymity because they were not authorized to speak publicly.

Last month, President Biden announced that his administration would begin offering booster shots for all Americans beginning the week of Sept. 20 pending sign-offs from the FDA and the CDC. Americans were told they should plan to get a third shot eight months after they received their second dose of Pfizer-BioNTech or Moderna vaccine. Biden said people who received the Johnson & Johnson vaccine would probably need a booster shot as well, but regulators would need more time to analyze data.

As new coronavirus variants emerge and the longevity of vaccine protection remains unknown, scientists are researching how booster shots could work. (John Farrell/The Washington Post)

Now, Woodcock and Walensky, who have faced criticism for endorsing a plan before their agencies completed their reviews, have warned that staff may need more time to make a determination about boosters for people who received the Moderna vaccine. The FDA has only partial data on Moderna and Johnson & Johnson boosters. Pfizer and BioNTech submitted late-stage booster data late last month.

“We always said we would follow the science and this is all part of a process that is now underway,” Chris Meagher, a White House spokesman, said in a statement. “We are awaiting a full review and approval by the FDA and a recommendation by the [CDC’s Advisory Committee on Immunization Practices]. When that approval and recommendation are made, we will be ready to implement the plan our nation’s top doctors developed so that we are staying ahead of this virus.”

More than 211 million doses of the Pfizer-BioNTech vaccine have been administered in the United States, according to the CDC. That compares with almost 146 million doses of Moderna and 14 million doses of Johnson & Johnson.

The FDA and the CDC declined to comment. The New York Times first reported the warnings.

In an interview Thursday with WebMD, an online publisher of health news, Woodcock said the agency did not have all the data it needs but that it was clear protection from vaccines wanes.

“You could talk to me in two months and if a plan hadn’t been made, you could be saying, well, why was there no contingency plan” if officials were becoming increasingly concerned, she said.

The issue of boosters has been fraught for months, punctuated by fierce debate about whether they are needed and when they should be administered. Critics outside the agency say the announcement that boosters might be available close to Sept. 20 placed too much pressure on the FDA and the CDC to get their reviews done quickly.

“I was really relieved to hear that they are going to take more time reviewing the data,” said Celine Gounder, an epidemiologist and member of Biden’s coronavirus transition task force. “There was some concern that they were jumping the gun for additional doses for the general population.” She said there is enough data to justify boosters for “very specific groups, but outside of that there’s not really compelling data.”

She added: “It’s almost like they were treating the anxiety rather than the public health issue.”

This week, news broke that two top vaccine officials, including the head of the FDA’s vaccine office, will retire this year. Those familiar with the situation said the career officials were frustrated with the White House announcement on boosters, but it is unclear whether that led to their exit.

Other experts argue the agencies were not always as fast as they needed to be, considering the urgent need to slow the spread of the highly transmissible delta variant. Data has shown vaccines lose some anti-virus potency over time, while also suggesting booster shots are effective at bolstering the immune response.

Officials involved in last month’s booster announcement said it was the culmination of intensive discussion among top doctors across the administration. The physicians grew increasingly concerned about rising cases, hospitalizations and particularly the growing number of breakthrough infections. After months of falling behind the curve on the pandemic, officials wanted to lay out a plan for boosters, with caveats about regulatory approval, because they thought the data was clear on waning immunity.

“Everyone wants to be thorough,” said one official involved in the process, who spoke on the condition of anonymity to describe internal discussions. “Everyone wants to assure safety, but it can’t be business as usual in the middle of a pandemic.”

But on Friday, some people familiar with the situation described the Sept. 20 date as “aspirational” — an aspiration that the FDA realized this week it wouldn’t be able to fulfill, at least for the Moderna and Johnson & Johnson products. The Moderna data was more complicated than initially expected and required more analysis, the individuals said.

Top infectious-disease expert Anthony S. Fauci on Sept. 5 said Pfizer booster shots will likely be ready by Sept. 20, but Moderna's could be delayed. (Reuters)

One issue involves the proposed dose for the third Moderna shot. The company said in a news release Wednesday it had begun submitting booster data to the FDA based on the results of a Phase 2 clinical trial involving 344 people. Six months after their second shot, those participants received a 50-microgram dose, half the amount used for the first two shots. Antibody levels, which had been waning at six months, increased after that booster, including among people 65 and older, the company said.

But officials aren’t sure whether the 50-microgram dose is the best to use and have indicated they need to understand the data better, according to the individuals. Some wonder if the original 100-microgram dose would offer stronger protection.

Those people also said the FDA had repeatedly expressed qualms about the Biden administration setting a target date for the planned start of a booster rollout, especially because the administration wanted it to include Moderna and possibly Johnson & Johnson, in addition to Pfizer-BioNTech. The agency had indicated it probably would not be ready to clear those boosters until October. But the individuals with knowledge said others in the administration wanted an earlier start date and that the week of Sept. 20 represented a compromise.

One person said Woodcock signed off on the Sept. 20 date before officials realized that some of the needed vaccine data was more complicated and coming in later than expected. The agency also wants to review raw vaccine data from Israel, which has begun giving booster shots to everyone 12 and older, to see if the FDA agrees with the conclusions of Israeli health authorities. Israeli officials have said the effectiveness of the Pfizer-BioNTech vaccine wanes six months after the second dose, and that a booster sharply increases protection.

The FDA is scheduled to meet with its outside vaccine experts to discuss Pfizer-BioNTech’s booster shot on Sept. 17.

Johnson & Johnson said it is working with health authorities in the United States and Europe on boosters and that it “continues to diligently generate and evaluate data from ongoing trials as well as emerging real-world evidence.”

Moderna did not immediately respond to a request for comment.