In a vote of 16-2, a key Food and Drug Administration (FDA) panel rejected a plan to approve Pfizer booster shots.
The Associated Press reports that the panel voiced frustration that Pfizer had provided little data on the safety of the extra doses.
President Joe Biden had set a target date of Sept. 20 to begin distributing a third dose of the vaccine.
In data released by the FDA on Wednesday, Pfizer argued that vaccine booster shots were not only safe and effective but would be necessary for all Americans over 16 because real-world studies show the effectiveness of the first two doses wanes over time.
The data also showed that a third vaccine dose restored efficacy in preventing COVID-19 to up to 95%.
In August, the Biden administration health officials recommended that those who received mRNA vaccines made by Pfizer and Moderna should seek out a booster eight months after their final shot — pending final approval from the FDA.
Earlier this month, Dr. Anthony Fauci said that it was likely that Pfizer's booster shots would likely be approved first and that Moderna's booster shot would follow a few weeks after the Sept. 20 target date.
The FDA has already approved the use of booster shots in people with compromised immune systems.