The US’ booster shot campaign against Covid-19 will likely start with only the Pfizer vaccine from September 20, the country’s top infectious disease expert has said.

The rollout for Moderna’s booster vaccine could, however, be delayed, CNBC news quoted Anthony Fauci as saying on Sunday.

Although both drugmakers have applied to US Food and Drug Administration (FDA) for authorisation of a third dose either six months or eight months after getting the second dose, they haven’t been officially approved by the agency.

Even as the approvals are pending, the Joe Biden administration has announced plans to offer third doses to people who received the Pfizer and Moderna shots.

“Only the Pfizer vaccine booster may get FDA and Centers for Disease Control and Prevention (CDC) approval in time for a rollout the week of September 20,” Fauci was quoted as saying on CBS’ aceFace the Nation.”

“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.”

He was not in favour of mixing doses and said that for people who got two doses of the Moderna vaccine, “it’s better to wait” for a third Moderna dose than get a Pfizer shot, CNN reported.

The booster doses of Pfizer and Moderna doses are based on CDC data that showed protection against infection waned several months after the second shot.

There was good reason to believe that a third dose “will actually be durable, and if it is durable, then you’re going to have very likely a three-dose regimen being the routine regimen,” Fauci was quoted as saying at a briefing last week.

An FDA advisory panel will review Pfizer’s application for a booster on September 17, only three days before shots are supposed to start, the report said.

Meanwhile, the FDA recently amended the emergency use authorisations for the Pfizer-BioNTech and Moderna Covid-19 vaccines to allow the administration of an additional dose to solid organ transplant recipients and other people who have an equivalent level of immunocompromise.

More than 1.3 million such immunocompromised individuals have received an additional shot, according to the CDC.