The U.S. will likely start to widely distribute Pfizer Covid-19 booster shots during the week of Sept. 20, but the rollout for Moderna’s vaccine could be delayed, White House chief medical advisor Dr. Anthony Fauci said Sunday.

The Biden administration has announced plans to offer third doses to people who received the Pfizer and Moderna shots, pending approval from public health officials. The U.S. recommends an additional shot eight months after the second dose.

Only the Pfizer vaccine booster may get Food and Drug Administration and Centers for Disease Control and Prevention approval in time for a rollout the week of Sept. 20, Fauci said on CBS’ “Face the Nation.” People who received Moderna shots may have to hold off for longer as the company waits for regulators to sign off on a third dose.

“Looks like Pfizer has their data in, likely would meet the deadline,” the director of the National Institute of Allergy and Infectious Diseases told CBS. “We hope that Moderna would also be able to do it, so we could do it simultaneously.”

“But if not, we’ll do it sequentially,” he continued. “So the bottom line is, very likely, at least part of the plan will be implemented, but ultimately the entire plan will be.”