Dr Anthony Fauci insisted he had a “great deal of confidence in what the UK does both scientifically and from a regulator standpoint," after suggesting British medicine experts had not assessed the Pfizer/BioNTech jab as carefully as their American counterparts.
On Wednesday, the UK became the first country in the world to approve the vaccine, which is up to 95 per cent effective at fighting Covid-19.
The British medicines regulator has insisted that it approved the jab without compromising safety.
Dr Fauci initially responded to the announcement by telling US broadcasters that American regulators would do a “more thorough job” than the British who appeared to have “rushed” their assessments.
Dr Fauci told CBS News that Britain “kind of ran around the corner of the marathon and joined it in the last mile” adding: “They really rushed through that approval.”
He also claimed that the speed of any approval could damage confidence, telling Fox News: “If you go quickly and you do it superficially, people are not going to want to get vaccinated.
“We have the gold standard of a regulatory approach with the FDA. The UK did not do it as carefully and they got a couple of days ahead.”
However, he later apologised for the remarks, saying there had been a “misunderstanding”.
He told the BBC: “Our process is one that takes more time than it takes in the UK. And that's just the reality.
“I did not mean to imply any sloppiness even though it came out that way."
The Medicines and Healthcare products Regulatory Agency (MHRA) has staunchly rejected any suggestion it sacrificed rigour for speed in its analysis of the vaccine.
“We have rigorously assessed the data in the shortest time possible, without compromising the thoroughness of our review,” it said in a statement.
“Covid-19 vaccines, including this one, are being developed in a co-ordinated way that allows some stages of this process to happen in parallel to condense the time needed, but it does not mean steps and the expected standards of safety, quality and effectiveness have been bypassed.”
It added a rolling review of the vaccine data started at the beginning of October, and the agency said it had made good progress on the review before the last submission of data was received.
“Any vaccine must undergo robust clinical trials in line with international standards, with oversight provided by the Medicines and Healthcare products Regulatory Agency.
“No vaccine would be authorised for supply in the UK unless the expected standards of safety, quality and efficacy are met.”
The UK has ordered 40 million doses of the Pfizer jab, enough to vaccinate 20 million people, with 800,000 doses expected to arrive by next week.