The Biden administration last month announced that a third dose of mRNA Covid-19 vaccines would be available to the public starting the week of Sept. 20—but now, public health officials are saying that timeline may be delayed in order for FDA and CDC to gather safety and efficacy data.

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Why the booster shot plan may be delayed

Acting FDA Commissioner Janet Woodcock and CDC Director Rochelle Walensky on Thursday told White House officials they might not have enough safety and efficacy data on booster shots to meet the Sept. 20 timeline, Politico reports.

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While the officials believe they have enough data to make booster doses of the Pfizer-BioNTech vaccine available by late September, Moderna's data is not yet sufficient, Axios reports.

Specifically, Moderna has submitted data to FDA on 50-microgram doses of booster vaccines, but FDA wants to see data on 100-microgram doses as well to evaluate which dosage provides a better immune response, a senior Biden administration official told Axios.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical adviser to the White House, said the administration was "hoping that we would get … both products, Moderna and Pfizer, rolled out by the week of the 20th. It is conceivable that we will only have one of them out, but the other would likely follow soon thereafter."

Fauci added he believes the delay "is going to be at the most, a couple of weeks' to a few weeks' delay, if any," and that the delay is a result of the administration wanting "the appropriate FDA regulatory approval and then the recommendation from the Advisory Committee on Immunization Practices."

"We always said we would follow the science, and this is all part of a process that is now underway," White House spokesperson Chris Meagher, said, adding the administration is waiting for a "full review and approval" of booster shots by FDA and a recommendation from CDC.

Meanwhile, it's unclear when Johnson & Johnson (J&J) will apply for booster shots of its vaccine, Politico reports. However, health officials have said J&J has more time to act before boosters are needed because its vaccines weren't available in the United States until well after Pfizer's and Moderna's were.

Health officials concerned about potentially waning protection

Some administration health officials have said the timeline for getting booster shots to the public was a result of concerning data regarding the vaccines' waning protection against symptomatic cases of Covid-19. Although the vaccines currently remain highly protective against severe cases that lead to hospitalization or death, officials worry that protection could also wane over time.

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Surgeon General Vivek Murthy on Thursday said, "Our feeling was that if we waited several more months, we would see protection against hospitalizations and deaths break down."

Similarly, Woodcock last week said the trend of breakthrough infections led officials to think "we are going to see hospitalizations and more serious disease" among the fully vaccinated at some point in the future. If that happens, Woodcock said, "we want to be ready" with a plan for booster shots.

However, some immunologists have said the evidence is not yet clear that boosters will be needed to avert severe disease outcomes.

Ali Ellebedy, who studies immune response to infection and vaccination at Washington University in St. Louis, said people he's studying are still generating an immune response to the second dose of their vaccine, even six months after receiving it.

"I don't think that will be hurt by additional immunization. I just think that well, we really didn't even reach the full potential of the second dose [yet]," Ellebedy said.

Ellebedy added that he's unsure whether a booster dose will significantly reduce the risk of severe Covid-19 in people who have received two doses of a vaccine.

"The immune memory we are seeing is really robust," he said. (Owermohle/Banco, Politico, 9/3; LaFraniere/Weiland, New York Times, 9/3; Frazier, Axios, 9/3; Branswell, STAT News, 9/2; Saric, Axios, 9/5; Owens, Axios, 9/4)