LONDON – The first 800,000 commercial doses of Pfizer Inc./Biontech SE’s COVID-19 vaccine are due to arrive in the U.K. over the next few days, after the Medicines and Healthcare products Agency (MHRA) became the first regulator to grant conditional approval.

A total of 1,500 immunization centers in the U.K. are prepared to receive the vaccine, with administration now expected to start on Dec. 7.

The vaccine, BNT-162b2, becomes the first messenger RNA (mRNA) drug to be approved, leapfrogging the multiple programs in which mRNA is being developed as the basis of cancer vaccines.

It has been a remarkably swift development, with the product progressing from a standing start when the SARS-CoV-2 genetic sequence was made public on Jan. 12, 2020, to the point where alongside MHRA, both FDA and EMA are poised to give emergency approval this month. Pfizer and Biontech said they will deliver 500 million doses before the end of 2020 and 1.3 billion doses in 2021.

Chinese and Russian regulators have both approved COVID-19 vaccines, but that has been on the basis of phase II data.

The U.K has a contract for 40 million doses in 2020-2021, the EU for 200 million doses with an option for a further 100 million and the U.S. has a contract for 100 million doses, for which it will pay $1.95 billion, with an option for a further 500 million doses. Pfizer said that pending approvals, the vaccine will be distributed pro rata, on the basis on population size.

MHRA’s decision is based on a rolling submission, including data from the phase III, in which BNT-162b2 was 95% effective in preventing people from contracting the infection from seven days post the second injection. All the agencies have seen the same data and have been assessing the different packages over several months. The fact that MHRA is first to grant an approval is down “to differences in the underlying processes” rather than different standards in scrutinizing the data, said Ben Osbourn, Pfizer U.K. managing director.

“The rest of the world is looking at the U.K. and [at] accelerating approval,” said Rene Reinert, professor of Microbiology and Infectious Diseases, who is vice president for Medical and Scientific Affairs at Pfizer Vaccines. “When you sent an email to MHRA you got a reply in 10 minutes; you were quick in responding,” Reinert said a U.K. press briefing.