In a new analysis from the FDA, the agency questions the strength of the evidence Johnson & Johnson presented on the company's application for booster shots.
The agency suggests, according to a document released ahead of a Friday meeting of the agency's vaccine advisers, that a key test used by the pharmaceutical giant was likely not sensitive enough to make their testing determinative.
It is unclear whether the agency will approve Johnson & Johnson's application. However, the decision will ultimately impact whether the 15 million Americans who received the one-dose vaccine will be allowed to receive a second J&J shot, or be advised to get a different brand of vaccine.
At present, Johnson & Johnson is recommending a booster at six months or later, "based on the strength of immune responses."
The Food and Drug Administration, however, is saying that the test the company used to measure the immune response of a six-month booster shot was not sensitive enough for the task.
"Although not independently confirmed by F.D.A. from datasets, summaries of the data suggest there may be a benefit in a second dose administered approximately 2 months after the primary dose," reads the FDA report.
Friday's meeting will include a presentation from federal scientists about mixing different brands of vaccines in order to provide boosts to the immune system against the coronavirus.
The FDA's determination about the J&J booster will likely have long-term effects on the use of the company's vaccine. If the agency recommends using a different brand of booster, it is unclear what purpose the J&J shot will continue to serve to the American public, which already overwhelming has opted to take the Pfizer and Moderna vaccines.
The FDA has already authorized an additional shot of the Pfizer vaccine for individuals aged 65 and up, and Moderna's application for a booster will be reviewed by the vaccine adviser panel on Thursday.