The vaccine developed by the University of Oxford stops 70% of people developing Covid symptoms, a large scale trial shows.
It is both a triumph and a disappointment after vaccines by Pfizer and Moderna showed 95% protection.
However, the Oxford jab is far cheaper, and is easier to store and get to every corner of the world than the other two.
So it will still play a significant role in tackling the pandemic, if it is approved by regulators.
The Oxford researchers have performed the normally decade-long process of developing a vaccine in around 10 months.
The UK government has pre-ordered 100 million doses of the Oxford vaccine, enough to immunise 50 million people.
What did the trial show?
More than 20,000 volunteers were involved, half in the UK, the rest in Brazil.
There were 30 cases of Covid in people who had two doses of the vaccine and 101 cases in people who received a dummy injection.
The researchers said it works out at 70% protection.
Intriguingly, the effectiveness rose to 90% in a group of volunteers who were given an initial half dose, followed by a full dose. It's not clear why there is a difference.
When will I get it?
In the UK there are four million doses ready to go, with another 96 million to be delivered.
But nothing can happen until the vaccine has been approved by regulators who will assess the vaccine's safety, effectiveness, and that it is manufactured to high standard. This will happen in the coming weeks.
However, the UK is ready to press the go button on an unprecedented mass immunisation campaign that dwarfs either the annual flu or childhood vaccination programmes.
Care home residents and staff will be first, followed by healthcare workers and the over 85s. The plan is to then work the way down the age groups.