Pfizer is expected to brief top U.S. government health officials in the coming days about the need for a coronavirus vaccine booster shot after an unusually public spat between the pharmaceutical giant and federal officials over whether a third shot will be necessary, according to the company and six people familiar with the plans.

Pfizer and the German firm BioNTech announced on Thursday that they planned to seek regulatory approval for a booster within weeks because they anticipated that people would need a third dose six to 12 months after receiving the companies’ two-shot regimen.

But top U.S. health officials have not decided whether boosters will be necessary. And several hours after the companies announced their intentions Thursday, the Department of Health and Human Services issued an emphatic rebuke, saying “Americans who have been fully vaccinated do not need a booster shot at this time.”

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

The debate about a booster shot comes as global anxiety rises about the spread of the highly transmissible delta variant, which has become the most prevalent strain in the United States. It also reflects ongoing concerns about vaccine hesitancy and the fairness of potentially giving Americans a third shot when much of the world lacks access to vaccines.

Pfizer has publicly cited data from Israel showing that its vaccine remains effective against the delta variant in preventing hospitalizations and serious illness but has declining effectiveness at preventing milder cases. The company is most focused on whether boosters are needed for the elderly and most vulnerable.

Pfizer said it has its own data showing that immunity against the coronavirus declines in vaccinated people over time, particularly in the elderly, but some scientists and health officials have cautioned that only the company has seen that data so far. The company will share the data with the FDA.

Those invited to the upcoming briefing include Anthony S. Fauci, President Biden’s chief medical adviser and director of the National Institute of Allergy and Infectious Diseases; Francis S. Collins, director of the National Institutes of Health; Rochelle Walensky, director of the Centers for Disease Control and Prevention; Janet Woodcock, acting commissioner for the Food and Drug Administration; David Kessler, chief science officer for the covid-19 response; and Vivek H. Murthy, the surgeon general. It is not yet clear who will attend. The meeting is scheduled for Monday, though that could change.

The people familiar with the meeting and others who described discussions about booster shots spoke on the condition of anonymity because they were not authorized to publicly discuss the issue. Several companies have regularly briefed U.S. officials on their data and findings throughout vaccine trials.

Pfizer will give U.S. officials a briefing similar to what they presented to Europeans last week about the need for a booster.

HHS and FDA declined to comment. A Pfizer representative confirmed the meeting but declined to provide details.

The HHS statement released after Pfizer’s Thursday announcement did not mention Pfizer by name but said “a science-based, rigorous process” led by the CDC, FDA and NIH would determine if boosters are necessary. The decision, the statement said, will be only partly informed by data from drug companies. It underscored that current data shows fully vaccinated people are protected against severe illness, even from the most challenging variants.

U.S. officials said they have not seen persuasive data clearly indicating boosters will be needed and are waiting to see whether Pfizer will have new or additional data that makes a compelling case. There is also concern that discussion of boosters could impact vaccine hesitancy, though two officials stressed that was not the main issue in deciding whether to recommend that some Americans receive a third shot.

Fauci told CNN on Friday that Pfizer CEO Albert Bourla called him to apologize for not alerting the U.S. government that it planned to seek authorization for a third shot of its vaccine.

Several senior health officials said they think it will probably be appropriate to recommend boosters for the elderly and the most vulnerable, such as those who are immunocompromised, but stressed there will be a public process to make such an evaluation, according to two senior administration officials.

An advisory panel to the CDC, called the Advisory Committee on Immunization Practices, will evaluate Pfizer’s data if the FDA authorizes Pfizer’s booster shot and recommend who should receive it, as the panel has done for several other health decisions related to coronavirus vaccines.

That panel heard a detailed presentation last month on what is known about the need for boosters and the questions that will need to be answered as authorities assess whether the United States is going to need additional doses in the future. Among the questions: Will boosters be needed, and if so, are they needed for everyone, or only in specific populations? What is the optimal timing, and can they be given as a “mixed dose,” or do they need to be matched with the original series of shots?

The FDA and the CDC advisory group will require data on safety, the immune response generated by boosters and public health, said Sara Oliver, a CDC official who is part of the advisory committee’s covid-19 work group.

The work group recommended that booster doses begin only after evidence of declining vaccine effectiveness against illnesses and signs that variants substantially affected vaccine protection. Oliver said there were no data to support recommendations for booster doses at the moment but that CDC is continuing to monitor vaccine effectiveness and the risk of variants.

Top health officials are still trying to understand the clinical significance of declining immunity in older people who have been vaccinated and whether that justifies administering a third shot.

Pfizer officials have begun to worry that data out of Israel showing declining effectiveness could cause some people to believe its vaccine is not effective. They have also grown concerned there could be outbreaks in nursing homes and among other vulnerable populations as the delta variant surges in the United States, and have worried about waiting too long to administer boosters, according to a person familiar with some of the discussions.

Receiving an emergency authorization from the FDA, and then a recommendation from the Advisory Committee on Immunization Practices, would probably take months. Pfizer officials are anxious that the process will take too long if U.S. officials do not take action soon.

Some scientists and health experts have said Pfizer stands to benefit financially from the administration of booster shots and said the company sparked confusion with its Thursday announcement.

All the pharmaceutical companies involved in making coronavirus vaccines are working on formulating and testing booster shots to prepare for the possibility that immunity fades or a new variant capable of totally escaping vaccines emerges — steps that experts agree are prudent and wise.

Some other countries have already laid out plans to begin administering booster shots. In the United Kingdom, the National Health Service has outlined plans for booster shots to begin in September, targeted first to groups that include people who are immunosuppressed, people older than 70 and front-line health-care workers. Eventually, its boosters will be offered to people older than 50 and adults at high risk. Israeli health officials are still considering whether to offer booster shots.

Pfizer said it would submit data to regulators within weeks showing that a booster at six months — a third shot of its original vaccine — caused antibody levels to shoot up five to 10 times higher than the original two-dose regimen. Moderna announced data from similar trials in May.

The drug company also announced it would begin testing a booster shot specifically programmed to combat the delta variant in August.

Carolyn Y. Johnson contributed to this report.