President Joe Biden's chief medical adviser said U.S. booster shots against COVID-19 are likely to start only with the vaccine by Pfizer Inc. and BioNTech SE, while the Moderna Inc. shot may be delayed.

"The bottom line is very likely at least part of the plan will be implemented, but ultimately the entire plan will be," Anthony Fauci said Sunday on CBS's "Face the Nation."

Fauci's comments may lead to more clarity on the administration's stance after Biden ran into resistance by medical experts who advise U.S. regulators over what they view as political interference in the review process.

While Biden has set a Sept. 20 target for kicking off the booster campaign, safety and efficacy data require signoff by the U.S. Centers for Disease Control and Prevention and the Food and Drug Administration.

Top medical officials, including CDC head Rochelle Walensky, warned the White House last week that regulators may only be able to act on the Pfizer shot, and possibly for just some groups of people, in the coming weeks, the New York Times has reported.

Fauci said Moderna "is getting their data together" and may have submitted it by now. Any delay for Moderna would be "a couple of weeks - if any," he said.

The company said Friday it had completed its submission to U.S. regulators.

Maryland Gov. Larry Hogan, a Republican, criticized the Biden administration for "mixed messaging."

"We need clear guidance on these booster shots because it undermines the credibility of it," Hogan said on NBC's "Meet the Press." "I guess they slipped and pre-leaked an announcement about booster shots with all three vaccines and then had to backtrack it and say you can only use Pfizer."

White House Chief of Staff Ron Klain pushed back against criticism that the Biden administration is rushing booster shots ahead of the scientific evidence.

CDC and FDA officials were involved in setting the week of Sept. 20 target date for booster clearance, he said on CNN's "State of the Union."

"We are ready to go as soon as the approval for the boosters comes from the FDA and from the CDC," Klain said.

Fauci said plans for boosters involve using with the same brand as the original vaccination, but that mix-and-match studies are being done to determine of people who got one shot as the primary vaccine could take another as a booster.