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In a new analysis, the Food and Drug Administration questioned the strength of evidence that Johnson & Johnson provided in its application for booster shots. A key test used by the company was likely not sensitive enough, the agency suggested, adding that it did not have enough time to independently review much of the raw data from the trials.

The document, released before a Friday meeting of the agency’s vaccine advisers, could have significant influence on whether the 15 million Americans who have received the one-dose vaccine will be allowed to get a second shot or if they will instead be urged to get a different brand of vaccine for added protection.

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The authors of the report did not take a position on whether the agency should approve Johnson & Johnson’s application, and it was unclear whether the flaws they identified in the data would be considered significant enough to deny the authorization.

Despite the FDA’s questions about the data, some experts anticipated that the agency would clear Johnson & Johnson’s boosters anyway to meet the public’s demand. Once the agency authorized a booster from Pfizer-BioNTech last month, “the die was cast,” said John Moore, a virus expert at Weill Cornell Medicine.

The agency’s analysis follows a report released Tuesday in which Johnson & Johnson argued in favor of a booster, presenting data from a number of trials.

“A booster dose is recommended at 6 months or later, based on the strength of the immune responses,” the company wrote.

But Wednesday, the FDA said that the test used by the company to measure the immune response of a six-month boost — known as a psVNA assay — is not sensitive enough for the task. The agency also questioned whether the increase in immune response was as big as the data suggested.

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“It is likely that the results seen are due to the low sensitivity of the psVNA assay used,” the FDA stated in its report.

Regulators warned that it was difficult to compare results from the company’s six-month and two-month booster studies as a result.

Johnson & Johnson in a statement said it looked forward to discussing the data at the Friday meeting, which will also include a presentation from federal scientists about mixing different brands of vaccines to provide boosts to the immune system.

The FDA’s discussion this week of the Johnson & Johnson vaccine has big implications for the shot’s US future, said Jason Schwartz, an associate professor of health policy at the Yale School of Public Health. The vaccine was already unlikely to have much uptake in the country in the long run, he said. And if the FDA recommends a booster shot for Johnson & Johnson recipients of a different vaccine, he added, “it’s hard to see what would steer people to the J&J vaccine.”

The FDA has already authorized an additional shot of the Pfizer-BioNTech vaccine for people older than 65 or with health conditions or job exposures that put them at higher risk. Moderna has also submitted an application for a booster that will be evaluated by the FDA’s advisers Thursday. It may also win authorization, despite limited evidence that the protection provided by an initial two doses of Moderna is waning.

Regulators on Wednesday wrote that a single shot of the Johnson & Johnson vaccine “still affords protection against severe COVID-19 disease and death in the United States” but that the highest effectiveness estimates, including for severe COVID-19, were “consistently less than the highest effectiveness estimates” for the Moderna and Pfizer-BioNTech shots.

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When Johnson & Johnson’s vaccine was authorized in February, it had several advantages over the other two. As a single shot, it was more convenient than the two-dose formulation from Moderna and Pfizer-BioNTech. It also did not have to be frozen to stay viable. But it also provided less robust protection.

A clinical trial showed that one dose of J&J had an efficacy rate of 66 percent against moderate to severe COVID-19 worldwide and 74 percent in the United States. Its efficacy against either severe or critical disease was stronger, at 85 percent worldwide.

In its application for a booster, Johnson & Johnson included the results of another large-scale trial that began in November in which they gave half their volunteers a second dose two months after the first. The other half received a placebo.

In August, the company announced that in the portion of the trial that took place in the United States, the efficacy rose to 94 percent. But in its report, the FDA focused on the worldwide results in which the increase was more modest, rising to 75 percent.

Against severe to critical COVID-19, two shots had an efficacy of 100 percent. But regulators warned in the analysis posted Wednesday that there was little data from that trial on the delta variant, which now causes the vast majority of US infections.

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“The small number of accrued cases confirmed to be caused by the delta variant precludes any conclusion regarding efficacy against that variant,” they wrote.