The European Union has agreed to buy up to 300 million doses of the BioNTech-Pfizer coronavirus vaccine, after it showed strong results in trials.
Deliveries are expected to start by the end of this year, the companies said, reports the BBC.
But the EU refused to provide details on how the vaccine would be rolled out, insisting that "a number of steps" needed to be followed beforehand.
Early data suggests the vaccine protects more than 90% of people from developing Covid-19 symptoms.
On Monday, its developers, Pfizer and BioNTech, said it had been tested on 43,500 people with no safety concerns raised, reports the BBC.
The companies plan to apply for emergency approval to use it by the end of November - and a limited number of people may get the vaccine this year.
At a press conference on Wednesday, European Health Commissioner Stella Kyriakides said the EU vaccine deal was "extremely important" .
"The agreement means we are a step closer to what we set out to do - to have a broad and solid vaccine portfolio," she said, adding that it would only be distributed once it is "proven to be safe".
But Ms Kyriakides refused to detail a specific timeline for when the vaccine would be delivered to member states. "It has to receive authorisation from the European Medicines Agency," she said. "A number of steps need to be followed before we will actually be able to have a timeline."
She also urged caution, saying: "[The vaccine] will not be a silver bullet that will make the virus disappear overnight".
Also on Wednesday, the head of the EU's health agency said the first vaccinations in the bloc could take place early next year, reports the BBC.
"I think optimistically [the] first quarter next year, but I can't be more precise," Andrea Ammon, the director of the European Centre for Disease Prevention and Control, told the AFP news agency.
She added that current infection trends in Europe were "very, very concerning" and all indicators were "going in the wrong direction right now".
Countries around Europe are battling a second wave of infections, and Spain and Italy recorded their highest numbers of deaths in weeks on Tuesday. Both countries would be likely to benefit from a rollout of the vaccine early next year, reports the BBC.
BioNTech-Pfizer is the first pharmaceutical company to share data from the final stages of testing - known as a phase 3 trial.
But trial results are also due in the next few weeks on a vaccine being developed by the British drug manufacturer AstraZeneca and scientists at the University of Oxford, reports the BBC.
A Russian vaccine called Sputnik V has also produced encouraging data.
On Wednesday, Russian scientists announced that the Sputnik V vaccine had so far shown a 92% success rate. That data, however, has not been reviewed by outside analysts.