Dr Anthony Fauci, the top US infectious disease expert, has apologised for remarks that seemed to criticise the UK's vaccine approval process.
"I have a great deal of confidence in what the UK does both scientifically and from a regulator standpoint," Dr Fauci told the BBC on Thursday.
The UK on Wednesday became the first country in the world to approve the Pfizer vaccine for the coronavirus.
It has defended the rapid approval and said the jab is safe and effective.
Dr Fauci on Wednesday told Fox News the UK did not review the vaccine "as carefully" as US health regulators. He later told CBS News that the UK had "rushed" the approval.
On Thursday, he walked back the comments, and said there was "no judgement on the way the UK did it".
"Our process is one that takes more time than it takes in the UK. And that's just the reality," Dr Fauci told the BBC. "I did not mean to imply any sloppiness even though it came out that way."
Dr June Raine, the head of the UK medicines regulator, on Wednesday said that "no corners had been cut" in vetting the vaccine. The Medicines and Healthcare Products Regulatory Agency (MHRA) reviewed preliminary data on the vaccine trials dating back to June.
"No vaccine would be authorised for supply in the UK unless the expected standards of safety, quality and efficacy are met," the regulator said.
On Thursday, the UK's deputy chief medical officer Prof Jonathan Van-Tam told the BBC he was "very confident" in the MHRA.
He said there was more than "100 years of medical experience" between the UK regulator and the committee advising which groups of people are vaccinated first.
Meanwhile, the first consignment of the Pfizer/BioNTech coronavirus vaccine has arrived in the UK.
It has been taken to a central hub at an undisclosed location, and will now be distributed to hospital vaccination centres around the UK.
The UK has ordered 40 million doses - enough to vaccinate 20 million people.
The vaccines are made in Belgium and have travelled to the UK via the Eurotunnel.
"Not better, not worse, just differently' - Fauci
Dr Fauci's remarks come as the US surpasses 14 million total Covid-19 infections, with a recorded 274,648 deaths.
The top doctor has said he believed that the US would have vaccine approval soon.
The FDA plans to meet on 10 December to discuss approval for the UK-approved vaccine, which was created through a partnership between Pfizer and BioNTech.
They will meet again on 17 December to discuss a second vaccine - Moderna - request.
Dr Fauci had described the US Food and Drug Administration's approval process, slower than the UK, as the "gold standard". On Thursday he clarified, saying the US does "things a little differently" than the UK.
"That's all," he said. "Not better, not worse, just differently."
The EU, meanwhile, is eyeing a 29 December meeting of the European Medicines Agency to determine if there is adequate safety data on the vaccine for it to be approved in Europe. This timeline puts the EU weeks behind both the UK and US. After the agency approves the vaccine, it will probably also need a sign-off from the EU Commission.
Also on Thursday, Dr Fauci met members of President-elect Joe Biden's team to discuss the incoming administration's response to the pandemic.
Biden later told CNN that he would retain Dr Fauci as chief medical adviser to the new administration's Covid-19 team.
Dr Fauci told CBS earlier that he agreed with Biden that it was "possible" the US might see an additional 250,000 deaths by January.