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.
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.
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.
European Health Commissioner Stella Kyriakides described the EU vaccine deal as "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 27-member bloc could take place early next year.
"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".
As countries around Europe battle a second wave of infections, Spain and Italy have recorded their highest numbers of deaths in weeks.
On Wednesday, Italy's health ministry reported 623 deaths - the highest daily figure since 6 April. The total number of cases the country has reported since the pandemic began also surpassed one million.
There are serious concerns over the country's ability to cope and some intensive care units are nearing capacity, the BBC's Mark Lowen reports from Rome. Pressure on the authorities is growing amid calls for a stricter lockdown to be introduced, but the government has so far resisted these demands.
Italy, Spain and other EU member states would likely benefit from a rollout of the BioNTech-Pfizer vaccine early next year.
The pharmaceutical companies are the first 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. 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.