The University of Oxford has developed a coronavirus vaccine that appears safe and triggers an immune response in humans, BBC reported.
A total of 1,077 people involved in trials showed the injection led to antibodies and T-cells to be made that can fight coronavirus.
It is still too early to tell 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 vaccine work?
The vaccine, developed at unprecedented speed, is called ChAdOx1 nCoV-19.
It is made from a genetically engineered virus that causes the common cold in chimpanzees.
It has been heavily modified to make it "look" more like coronavirus but it cannot cause infections in people.
Scientists transferred the genetic instructions for the coronavirus's "spike protein" to the vaccine they were developing to create the resemblance.
The "spike protein" is the crucial tool the virus uses to invade our cells.
This means the vaccine resembles the coronavirus and the immune system can learn how to attack it.
What are antibodies and T-cells?
Antibodies have so far been the focus on Covid-19 so far.
But antibodies are small proteins made by the immune system that stick onto the surface of viruses and are only one part of the human body's immune defence.
The coronavirus can be disabled by neutralising antibodies.
T-cells, on the other hand, are a type of white blood cell.
They help co-ordinate the immune system by spotting 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.
Antibody levels peaked after 28 days.
However, the study out in the Lancet has not run for long enough to understand how long they may last.
Side effects
There were no dangerous side-effects from taking the vaccine, but 70 per cent of people on the trial developed either fever or headache, but these can be managed with paracetamol.
The study still cannot show whether the vaccine can either prevent people from becoming ill or even lessen their symptoms of Covid-19 though.
More than 10,000 people will take part in the next stage of the trials in the UK.
It is possible a coronavirus vaccine will be proven effective before the end of the year, but it will not be widely available.
Health and care workers will be prioritised, regardless.
The Oxford vaccine is not the first to reach this stage, with groups in the US and China also publishing similar results.
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