White House Chief Medical Advisor Dr. Anthony Fauci said Sunday that Americans will be getting the Pfizer COVID-19 vaccine’s third (booster) dose from September 20, according to CNN.

However, Dr. Fauci said the distribution of Moderna’s booster dose could be delayed.

Last month, the Biden administration announced that would offer boosters doses to people who received the Pfizer-BioNTech and Moderna vaccines after approval from public health officials, recommending the third shot eight months after the second dose.

Only the Pfizer-BioNTech vaccine’s booster dose may get approval from the Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) in time for a rollout during the week of September 20, Dr. Fauci told CBS’ Face the Nation.

However, those who have received the Moderna vaccine may have to wait longer as the company waits for the FDA and CDC to sign off on its booster dose.

Dr. Fauci, who is also the director of the National Institute of Allergy and Infectious Diseases (NIAID), told CBS, “Looks like Pfizer has their data in, likely would meet the deadline. 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 added. “So the bottom line is, very likely, at least part of the plan will be implemented, but ultimately the entire plan will be.”

Dr. Fauci told CNN that for people who received the second dose of Moderna vaccine, “it’s better to wait” for a third dose rather than getting a Pfizer dose.

He added that the administration plans to release data in the coming weeks on mixing vaccines from different manufacturers, according to CNN.

In the United States, Pfizer-BioNTech jab has been the most widely administered vaccine, with more than 95 million people receiving the full two-dose regimen, according to CDC. Around 66 million people have received the two-dose Moderna vaccine and 14 million received the one-dose Johnson & Johnson vaccine. However, the regulatory bodies have not announced a booster dose of the J&J vaccine.

However, the Biden administration has already approved a booster dose of Pfizer vaccine for immunocompromised people, meaning those who have a weakened immune system.

Experts have found that the efficacy of Pfizer and Moderna vaccines wanes after several weeks of the second dose, calling for a booster dose. The coronavirus continues to spread across the globe, mutating and giving rise to new and potentially more dangerous strains.