A new coronavirus vaccine could prevent more than 90% of people from getting Covid-19, according to early analysis of it.
The vaccine is being developed by pharmaceutical firms Pfizer and BioNTech and is currently in the final stage of testing.
It has been tested on 43,500 people in six countries and no safety concerns have been raised so far.
The companies behind the vaccine plan to apply for emergency approval to use the vaccine by the end of the month.
But scientists still aren't sure exactly how effective the vaccine is.
Although the developers say it can protect more than 90% of people, this figure is just based on early analysis of the first 94 volunteers who developed the coronavirus.
This means that when they analyse the full results, they may find that the vaccine is either more or less effective than they previously thought.
It's also not clear if the vaccine just stops people from getting the virus for a short time, or if it can make them immune in the long-term.
This vaccine uses a completely experimental approach - it involves injecting part of the virus's genetic code in order to train a person's immune system.
It is different to other more commonly used vaccines. They work by introducing a tiny bit of the disease into your body, so that your immune system knows how to fight it in the future.
This type of vaccine, called an RNA vaccine, is different. It takes advantage of the process that cells use to make proteins.
People need to take two doses of the vaccine, three weeks apart.
The trials - in the US, Germany, Brazil, Argentina, South Africa and Turkey - showed that 90% protection is achieved seven days after the second dose.
Researchers hope that by the third week of November they'll have enough data to take their vaccine to regulators.
The regulators will look at the trials and check that the vaccine meets the necessary standards and safety requirements, but until then no countries will be able to use it.
Pfizer believes it will be able to supply 50 million doses by the end of this year, and around 1.3 billion by the end of 2021.
The UK should get 10 million doses by the end of the year, with a further 30 million doses already ordered.
However storing and transporting the vaccine all over the world could also be difficult, as it has to be kept in ultra-cold storage below minus 80C.
There are still lots of questions - for example, how long does immunity last, does the vaccine work as well in high-risk elderly people, does it stop you spreading the virus or just from developing symptoms?
Most experts think the vaccine won't be widely available until the middle of 2021.
While many have seen it as positive news, there are also some calls for caution.
Dr David Nabarro, from the World Health Organization (WHO), said the vaccine would help - but warned that people would still have to follow rules, such as social distancing, for many months to come.
"I just want to stress that these principles that we have been working for are still absolutely essential even if a vaccine arrives in the next few months," he said.
Downing Street has described the news as "promising" with a statement from Prime Minister Boris Johnson saying that "the NHS stands ready to begin a vaccination programme for those most at risk, once a Covid-19 vaccine is available".
England's Chief Medical Officer, Professor Chris Whitty, has said it "demonstrates the power of science" in fighting the coronavirus. He added that "the final safety and efficacy data" needs to be seen, but that it is "very encouraging".
Dr Albert Bourla, the chairman of Pfizer, 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."
Prof Ugur Sahin, one of the founders of BioNTech, described the results as a "milestone".
However, Dr Sophie Harman, global health expert at Queen Mary University London, says although the news from Pfizer is "incredibly exciting" we shouldn't get "too carried away" and "really have to be careful about safety and scale around the vaccine".
She said there are questions about who gets the vaccine and who gets the vaccine first, and how many people will actually want to take up the vaccine if it is offered.
She added: "This cannot replace just the tried and tested methods of track and trace, monitoring the outbreak, and changing our behaviour."
There are around a dozen different vaccines in the final stages of testing, but this one is the first to show any results.
One of the leading projects is from The University of Oxford and AstraZeneca .
Prof Peter Horby, from the University of Oxford, said today's news about the Pfizer vaccine made him "smile from ear to ear".
He added: "It is a relief... there is a long long way to go before vaccines will start to make a real difference, but this feels to me like a watershed moment."
What do you want to know about this story? Do you have any questions about the search for a coronavirus vaccine? Let us know what they are, by commenting below.