The United States will likely start to widely distribute Pfizer (PFE  )-BioNTech (BNTX  ) COVID-19 booster doses during the week of Sept.20, with the rollout for the Moderna (MRNA  ) vaccine expected to be delayed, White House chief medical advisor Dr. Anthony Fauci said Sunday.

The White House recently announced plans to offer third doses of either vaccine to people that have completed the two shot regimen, with the plan hinging on approval from both the Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC).

Currently, the U.S. recommends for an additional dose at most eight months after the second dose; President Joe Biden recently said that researchers were testing booster shots for five weeks post second dose.

Fauci told CBS' "Face the Nation" on Sunday it is likely that only the Pfizer vaccine may get official approval in time for a booster shot rollout the week of Sept. 20, with those who received Moderna shots possibly having to wait longer as the biotech wait for regulatory approval for third dose boosters.

"Looks like Pfizer has their data in, likely would meet [the White House's] deadline. We hope that Moderna would also be able to do it, so we could do it simultaneously," Fauci said. "But if not, we'll do it sequentially. So the bottom line is, very likely, as least part of the plan will be implemented, but ultimately the entire plan will be [eventually.]"

The FDA's advisory panel is set to review Pfizer-BioNTech's booster shot application on Sept. 17, with CDC approval set to follow soon after if the FDA clears the booster dose for the public.

The U.S. has been working on approving booster doses after CDC data demonstrated that immune system protection against infection begins to wane several months after the second vaccine dose, especially for the Pfizer-BioNTech shot. However, both vaccines still provide protection against severe disease, hospitalization and death even after infection-protecting antibodies begin to diminish.

The Pfizer-BioNTech vaccine is the most administered COVID shot in the U.S., according to CDC data, with nearly 96 million Americans receiving a full two shot regime. Moderna comes second with more than 66 million people receiving two doses, while about 14 million Americans have received the one-shot Johnson & Johnson (JNJ  ) vaccine. U.S. regulators need more data before than can recommend a booster shot for those who were dosed with the Johnson & Johnson vaccine.

The World Health Organization (WHO) has criticized the Biden administration's plans to offer booster doses to Americans who are not immunocompromised, with the global health regulator going as far as condemning wealthy nations for stockpiling COVID vaccines while many lower-income nations are having trouble inoculating their vulnerable populations, like the elderly and health-care workers.

The U.S. has fully vaccinated about 53% of its population and has already administered booster shots to more than 1.3 million immunocompromised Americans, according to CDC data. By comparison, nations on the continent of Africa have vaccinated just 3% of their populations, according to the WHO.