The developers - Pfizer and BioNTech - described it as a "great day for science and humanity". Their vaccine has been tested on 43,500 people in six countries and no safety concerns have been raised. The companies plan to apply for emergency approval to use the vaccine by the end of the month. No vaccine has gone from the drawing board to being proven highly effective in such a short period of time.
Priminister Boris Johnson said the vaccine has "cleared a significant hurdle", but warned it was "very early days". At a Downing Street news conference, Mr Johnson warned people not to "rely on this news as a solution" to the pandemic.
A vaccine - alongside better treatments - is seen as the best way of getting out of the restrictions that have been imposed on all our lives.
How effective is the COVID-19 vaccine?
The data shows that two doses, three weeks apart, are needed. The trials - in US, Germany, Brazil, Argentina, South Africa and Turkey - show 90% protection is achieved seven days after the second dose. However, the data presented is not the final analysis as it is based on only the first 94 volunteers to develop Covid so the precise effectiveness of the vaccine may change when the full results are analysed.
The chairman of Pfizer, Dr Albert Bourla, said: "We are a significant step closer to providing people around the world with a much-needed breakthrough to help bring an end to this global health crisis."
When will the vaccine be available?
A limited number of people may get the vaccine this year.
Pfizer and BioNTech say they will have enough safety data about the vaccine by the third week of November. Then they can take their vaccine to regulators. Until the vaccine has been approved it will not be possible for countries to begin their vaccination campaigns.
The two companies said they will be able to supply 50 million doses by the end of this year and around 1.3 billion by the end of 2021. Each person needs two doses.
The UK should get 10 million doses by the end of the year, with a further 30 million doses already ordered.
Who would get it?
Not everyone will get the vaccine straight away. The countries are each deciding who should be prioritised to get the vaccine.
Hospital staff and care home workers are expected to be near the top of every list because of the vulnerable people they work with, as will the elderly who are most at risk of severe disease. The UK is likely to prioritise older residents in care homes and the staff that work there. People under 50 and with no medical problems are likely to be last in the queue.
A final decision has not been made yet, saying it will depend on how well the vaccine works in different age-groups and how the virus is spreading.
Are there any potential issues?
The vaccine appears safe from the large trials so far but nothing, including paracetamol, is 100% safe. And there are still many unanswered questions as this is only interim data.
We do not know if the vaccine stops you spreading the virus or just from developing symptoms. Or if it works equally well in high-risk elderly people. The biggest question - how long does immunity last - will take months or potentially years to answer.
There are also massive manufacturing and logistical challenges in immunising huge numbers of people, as the vaccine has to be kept in ultra-cold storage at below minus 80C.
How does it work?
There are around a dozen vaccines in the final stages of testing - known as a phase 3 trial - but this is the first to show any results.
It uses a completely experimental approach - that involves injecting part of the virus's genetic code - in order to train the immune system.
Previous trials have shown the vaccine trains the body to make both antibodies - and another part of the immune system called T-cells to fight the coronavirus.
Source - BBC