Britain's medicines regulator insisted Friday its met all safety standards, after officials in Europe and the United States raised questions about the rapid process.
Britain announced on Wednesday it had given emergency approval for the vaccine's general use and would start rolling it out next week.
"Any vaccine must undergo robust clinical trials in line with international standards, with oversight provided by the Medicines and Healthcare products Regulatory Agency," the MHRA regulator said in a statement.
"No vaccine would be authorised for supply in the UK unless the expected standards of safety, quality and efficacy are met."
Leading US infectious disease scientist Anthony Fauci on Thursday said the MHRA had "rushed through that approval" .
"I have a great deal of confidence in what the UK does both scientifically and from a regulator standpoint," Dr Fauci, who leads the US National Institute of Allergy and Infectious Diseases, told the BBC.
"Our process is one that takes more time than it takes in the UK. And that's just the reality," he said, adding: "I did not mean to imply any sloppiness even though it came out that way.
Britain plans to distribute an initial batch of 800,000 doses starting next week, prioritising care homes.
MHRA chief executive June Raine previously insisted that "no corners had been cut" in vetting the Pfizer-BioNTech vaccine.
With Britain transitioning out of the European Union, the UK regulator obtained an exemption from the EU's medicines agency, which has stressed it prefers to wait for further review and consultations across the bloc.
The MHRA said it was able to move quickly after launching a rolling review of data provided by Pfizer and BioNTech as human trials proceeded, rather than waiting for all the data to be compiled at the end of the trials.
"COVID-19 vaccines, including this one, are being developed in a coordinated 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 said.
Approval is expected this month by both the US Food and Drug Administration and the European Medicines Agency.
England's deputy chief medical officer Jonathan Van-Tam has also hit back at critics, suggesting sour grapes on the part of regulators who are bound by more cumbersome procedures.
"If you're a regulator who's slightly further behind, what do you say to justify your position that you are further behind? Words such as the ones we've heard perhaps," he told the BBC on Wednesday.
Some British ministers have strayed into anti-EU jingoism after London beat Brussels to approving the Pfizer-BioNTech jab.
But Dr Van-Tam also stressed the international nature of scientific collaboration.
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