Dr Fauci is concerned the people who haven't had the 'full impact' of two doses of vaccines will be dangerously vulnerable to infection with variants that weaken the effects of antibodies.
More than 40 percent of U.S. adults have now had at least one dose of vaccine, yet cases have been rising for four straight weeks.
'We know that these increases are due, in part, to more highly transmissible variants, which we are very closely monitoring,' said Centers for Disease Control and Prevention (CDC) director Dr Rochelle Walensky during a monday White House briefing.
Her warning underscored Dr Fauci's concern that Americans need to get both doses of vaccines made by Pfizer and Moderna on time, despite the fact that stretching these timelines have allowed the UK to speed ahead of the U.S. in the vaccination race.
Dr Anthony Fauci warned Monday that people who have only had one dose of Covid vaccines made by Moderna or Pfizer would have only 'tenuous' protection against variants
In the UK, the second dose of vaccines made by Pfizer or AstraZeneca may be delayed up to 12 weeks.
The strategy was controversial when it was first introduced there, but has helped the UK get nearly half of its population some level of protection with one or more dose of vaccine.
At the time, Dr Fauci rejected and even criticized the UK for employing the untested strategy (although he later walked back his harsh words).
But Monday he said there was no 'right or wrong' strategy, when it comes to delaying doses.
'There are are different approaches and different opinions,' he said.
'We have been concerned, and still are, that when you look at the level of protection after one dose, you can say is 80 percent, but it is somewhat of a tenuous 80 percent, because the level of, for example, neutralizing antibodies against the coronavirus, when you just leave it at one dose, the question is, how long does it last?'
Research published last month showed that the risk of contracting COVID-19 after a first dose of either Pfizer's or Moderna's vaccines was reduced by about 80 percent within two weeks of that first shot.
But a second dose was administered to those people one or two weeks later, depending on whether they got the Pfizer vaccine, which has a dosing regimen with a three-week gap, or Moderna's, which should be given four weeks later.
It remains unclear how protected people would be by one dose after that second week.
And with variants circulating, every bit of protection counts.
Two doses of either vaccine are still protective against vaccine-evading variants from Brazil and South Africa.
But the antibodies generated by these vaccines are between two-fold and six-fold less effective at binding to the spike protein that allows coronavirus to break into human cells.
So, to Dr Fauci's mind, it may not be worth the risk to leave more people with just a first dose for longer.
Neither variant is common in the U.S., so the odds that Americans will encounter them are slim.
So far, the South African variant and Brazil variant each account for fewer than 0.05 percent of Covid cases in the U.S, according to tracking from Outbreak.info.
Still, they are substantially more infectious than the 'wild-type' virus, and experts worry they will only become more prevalent.