The coronavirus vaccine being developed at Oxford University is safe and prompts a reaction in the immune system, trials show.
Researchers say the jab could provide double protection against Covid-19, but further studies are needed to prove it works.
The early stage trial found that the vaccine is safe and causes few side effects.
It also induces strong immune responses in both parts of the immune system – provoking a T cell response within 14 days of vaccination, and an antibody response within 28 days.
Compared with the control group of those given a meningitis vaccine, the Covid-19 vaccine caused minor side effects more frequently, according to the study.
But some of these could be reduced by taking paracetamol, the researchers said, adding that there were no serious adverse events from the vaccine.
More than 100m doses of the vaccine have already been ordered by the UK Government should it succeed.
Prime Minister Boris Johnson tweeted: “This is very positive news. A huge well done to our brilliant, world-leading scientists and researchers.
“There are no guarantees, we’re not there yet and further trials will be necessary – but this is an important step in the right direction.”
The findings on the Oxford vaccine trial were published in medical journal, The Lancet.
Professor Sarah Gilbert said: “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.
“As well as continuing to test our vaccine in phase-three trials, we need to learn more about the virus – for example, we still do not know how strong an immune response we need to provoke to effectively protect against Sars-Cov-2 infection.
“If our vaccine is effective, it is a promising option as these types of vaccine can be manufactured at large scale.
“A successful vaccine against Sars-Cov-2 could be used to prevent infection, disease and death in the whole population, with high-risk populations such as hospital workers and older adults prioritised to receive vaccination.”
An ideal vaccine against SARS-CoV-2 should be effective after one or two vaccinations and work in target populations including older adults and those with other health conditions, researchers say.
They add that it should confer protection for a minimum of six months, and reduce onward transmission of the virus to contacts.
However, the experts warn that the current trial is too preliminary to confirm whether the new vaccine meets these requirements.
Phase two – in the UK only – and phase three trials to confirm whether it effectively protects against the virus are taking place in the UK, Brazil and South Africa.
The trial included 1077 healthy adults aged 18-55 years with no history of Covid-19, and took place in five UK hospitals between April 23 and May 21.