"We are not going to have sufficient vaccinations in place to prevent a surge in cases for three to six months," Mike Ryan told a social media event, calling on people to maintain physical distancing and respect other measures to restrict the spread of COVID-19.

The UK approved Pfizer-BioNTech's COVID-19 vaccine on Wednesday, jumping ahead of the rest of the world in the race to begin the most crucial mass inoculation program in history with a shot tested in wide-scale clinical trials.

Prime Minister Boris Johnson touted the green light from Britain's medicine authority as a global win and a ray of hope amid the pandemic, though he recognised the logistical challenges of vaccinating an entire country of 67 million.

Britain's move raised hopes that the tide could soon turn against a virus that has killed nearly 1.5 million people globally, hammered the world economy and up-ended normal life for billions since it was first detected in Wuhan, China, a year ago.

The UK's Medicines and Healthcare products Regulatory Agency (MHRA) granted emergency use approval to the vaccine developed by Pfizer and German biotechnology partner BioNTech, which they say is 95 per cent effective in preventing illness, just 23 days after Pfizer published the first data from its final stage clinical trial.

"Fantastic news," Johnson told parliament, though he cautioned that people should not get carried away.

"At this stage it is very, very important that people do not get their hopes up too soon about the speed with which we will be able to roll out this vaccine."

The world's big powers have been racing for a vaccine for months and coming first may be seen as a coup for a government that has faced criticism over its handling of the crisis.

Both the US and EU regulators are sifting through the same Pfizer vaccine trial data but have yet to give their approval.

Britain's speed drew criticism from officials in Brussels where the EU regulator said its longer procedure was more appropriate as it was based on more evidence.

Just hours after the British announcement, President Vladimir Putin ordered Russian authorities to begin mass voluntary vaccinations next week.

Russia will have produced 2 million doses of its own vaccine within the next few days, Putin said.

Pfizer vaccine executive Ralf Rene Reinert said other countries were looking to emulate Britain.

"I wouldn't make this a race between countries," he said.

"The UK is leading, and now let's see how the other countries move forward. And Pfizer will do everything to exchange all the data packages with every regulatory agency."

German Health Minister Jens Spahn said EU member countries could have issued similar emergency approvals but the bloc had decided to move forward as one.

"It's very important we do this to help promote trust and confidence in this authorisation," he said.

The MHRA approved the vaccine in record time by doing a "rolling" concurrent analysis of data and the manufacturing process while Pfizer raced to conclude trials.

"No corners have been cut," MHRA chief June Raine said in a televised briefing from Downing Street, adding that the first data on the vaccine had been received in June and undergone a rigorous analysis to international standards.

US Health Secretary Alex Azar said that the UK move should inspire confidence.

"For the American people this should be very reassuring: an independent regulatory authority in another country has found this vaccine to be safe and effective for use," Azar told the Fox Business Network.

The US Food and Drug Administration will make a decision on emergency use authorisation on the Pfizer/BioNTech vaccine in days or weeks after a panel of outside advisers meets on December 10.

The FDA often but not always follows the panel's advice.

The European Medicines Agency said it could give emergency approval for the shot by December 29 while the World Health Organisation said it was reviewing it for "possible listing for emergency use".

Britain said it would start vaccinating those most at risk of dying early next week after it gets 800,000 doses from Pfizer's manufacturing centre in Belgium and UK Health Secretary Matt Hancock said tests on that first batch had been completed.

The speed of the roll-out depends on how fast Pfizer can manufacture and deliver the vaccine - and the extreme temperature of minus 70C at which the vaccine must be stored.

It can be kept for five days in a regular fridge.

Australian Associated Press