A coronavirus vaccine developed by the University of Oxford appears safe and triggers an immune response.
Trials involving 1,077 people showed the injection led to them making antibodies and T-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.
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.
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 co-ordinate 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 how long they may last, the study in the Lancet showed.
Prof Andrew Pollard, from the Oxford research group told the BBC: “We’re really pleased with the results published today as we’re seeing both neutralising antibodies and T-cells.
“They’re extremely promising and we believe the type of response that may be associated with protection.
“But the key question everyone wants to know is does the vaccine work, does it offer protection… and we’re in a waiting game.”
The study showed 90% of people developed neutralising antibodies after one dose. Only ten people were given two doses and all of them produced neutralising antibodies.
“We don’t know the level needed for protection, but we can maximise responses with a second dose,” Prof Pollard told the BBC.
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.

• BBC