Trials involving 1,077 people showed the injection led to them making antibodies and white blood cells that can fight coronavirus.
The findings are hugely promising, but it is still too soon to know if this is enough to offer protection and larger trials are under way.
The UK has already ordered 100 million doses of the vaccine.
How does the vaccine work?
The vaccine – called ChAdOx1 nCoV-19 – is being developed at unprecedented speed.
It is made from a genetically engineered virus that causes the common cold in chimpanzees.
It has been heavily modified, first so it cannot cause infections in people and also to make it “look” more like coronavirus.
Scientists did this by transferring the genetic instructions for the coronavirus’s “spike protein” – the crucial tool it uses to invade our cells – to the vaccine they were developing.
This means the vaccine resembles the coronavirus and the immune system can learn how to attack it.
What are antibodies and T-cells?
Much of the focus on coronavirus so far has been about antibodies, but these are only one part of our immune defence.
Antibodies are small proteins made by the immune system that stick onto the surface of viruses.
Neutralising antibodies can disable the coronavirus.
T-cells, a type of white blood cell, help coordinate the immune system and are able to spot which of the body’s cells have been infected and destroy them.
Nearly all effective vaccines induce both an antibody and a T-cell response.
Levels of T cells peaked 14 days after vaccination and antibody levels peaked after 28 days. The study has not run for long enough to understand what long-term immunity may look like.
Is it safe?
Yes, but there are side-effects.
There were no dangerous side-effects from taking the vaccine, however, 70% of people on the trial developed either fever or headache.
The researchers say this could be managed with paracetamol.
Prof Sarah Gilbert, form the University of Oxford, UK, says: “There is still much work to be done before we can confirm if our vaccine will help manage the COVID-19 pandemic, but these early results hold promise.”