Three shots will come to be viewed as the necessary requirement to be fully vaccinated against Covid-19, Dr Anthony Fauci has told The Telegraph.
The comments by America's top infectious diseases expert represented backing for the UK's decision to start rolling out booster shots to over-50s.
It came as the US Food and Drug Administration's advisory panel, in a shock move on Friday night, rejected a plan to offer Pfizer booster shots to Americans over the age of 16. They later voted in favour of authorising boosters of Pfizer's coronavirus vaccine for everyone aged 65 and over.
In an interview with The Telegraph, Fauci said: "I believe, when all's said and done, it's going to turn out that the proper regimen, at least for an mRNA vaccine [like Pfizer], is the two original doses, the prime followed in three-to-four weeks by a boost, but also followed several months later by a third shot.
"So I think that ultimately, when we look back on this, it's going to be that the proper regimen, to have a complete and full regimen, will be a third shot boost."
He added: "I'm favourably disposed to boosts based on the waning of immunity, as we're seeing very, very clearly as time goes on, in the data in the US [and] even more dramatic data from our Israeli colleagues.
"[There is] some indication of waning in the UK, which is the reason why the UK has, or will soon be, vaccinating people 50 years of age and older, as well as healthcare providers and those who are immuno-compromised."
Fauci said it was not yet clear if children would need a third shot. He said: "We don't know that. I think it's too early to make that determination."
Could babies be next?
As the UK approved 12- to 15-year-olds to have a single shot of the Pfizer vaccine, Fauci also detailed how he believed children as young as 5 in the US would be getting vaccinated by late autumn.
He said the vaccination of children aged between 6 months and 5 years would happen "likely some time after".
Fauci acknowledged that giving a Covid-19 vaccine to children "at that younger age is not uniformly agreed upon around the world". But he said: "We are planning, and are doing, the clinical studies right now to extend the vaccination.
"The data we are collecting on the children in the clinical trial, from 11 to five, will likely be available for evaluation by our regulatory authorities by mid-fall, I would say some time around October, November.
"So, it looks like we will be vaccinating children before the end of the year down to five-years-old, if our regulatory authorities give us the go-ahead."
Fauci said concerns about health impacts on children were "the reasons we do clinical trials".
The main concern with the Covid-19 vaccine has been myocarditis. But he said that was generally seen in children over 12 and into young adulthood.
He said: "Although we don't expect in the very younger children that we're going to see it, we're still going to do the studies for safety.
"My feeling is this, if we can establish the safety element of it, and the risk benefit ratio weighs heavily on the benefit, I certainly would be in favour of it.
"But I want to reserve my opinion until we see the data, and the regulatory authorities examine the data carefully."
Fauci supported masks mandates in schools saying, firstly, children should be "surrounded with people who are vaccinated".
He said: "But that doesn't necessarily seem to be enough so, superimposed upon all that, what we've done in the US is strongly recommend there be mask mandates."
Fauci said mandating vaccines for children was also a "subject of considerable controversy and concern and is still being discussed. There has been no decision on it."
But he said: "You know, mandating vaccines for kids in schools is not a new phenomenon. We do that all the time with other vaccines, for example measles, mumps, rubella, hepatitis, polio.
"So, the idea of mandating vaccines for children in schools is not unique to Covid-19. It depends on the risk to children.
'We're in the middle of a devastating outbreak. We've had 650,000 deaths in the United States, and we're still having about 120,000 new infections a day. We're in the middle of a very serious situation here in the United States."