The early stage trial, which involved 1,077 people, has found that the vaccine trains the immune system to produce antibodies and white blood cells capable of fighting the virus. It also causes few side effects.
Professor Sarah Gilbert, co-author of the Oxford University study, described the findings as promising but said there “is still much work to be done before we can confirm if our vaccine will help manage the Covid-19 pandemic”.
The results came as the UK secured 90 million doses of other promising Covid-19 vaccines, while clinical trials of a new inhaled coronavirus treatment showed it significantly reduced the number of hospitalised patients needing intensive care.
Boris Johnson, the prime minister, said the results were ”very positive” as he congratulated the team of scientists working on the vaccine.
“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,” he said on Twitter.
Matt Hancock, the health secretary also told MPs the virus was "on the back foot" in Britain as the number of new cases and hospital admissions continue to fall.
In research published on Monday in the journal Lancet, scientists said the vaccine produced a dual immune response in people aged 18 to 55 – provoking a T-cell response within 14 days of vaccination and an antibody response after 28 days.
More trials will need to be conducted to establish how long these cells last within the body. It is also unclear whether the vaccine can prevent people from falling ill or lessen the symptoms of Covid-19.
Researchers said the vaccine caused minor side effects more frequently than a control group, but some of these could be reduced by taking paracetamol, with no serious adverse events from the vaccine.
The data included in the paper covered the first 56 days of the trial and is ongoing.
Phase two and three trials evaluating the vaccine’s effectiveness have been expanded beyond the UK to South Africa, Brazil and the US, where infections rates are still high, to allow researchers to assess whether it can block or limit transmission among large populations.
“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,” Professor Gilbert added.
The study’s authors have also noted that more research is needed to confirm the findings in different groups of people – including older age groups, those with other health conditions, and in ethnically and geographically diverse populations.
The vaccine, called AZD1222, is made from a genetically engineered virus that causes the common cold in chimpanzees.
It has been weakened to ensure it does not trigger any disease in humans and is modified to express the spike protein seen on Sars-CoV-2 – the tool used by the virus to invade human cells.
When this genetic material enters the human body, it “helps teach the immune system to recognise the Sars-CoV-2 virus”, according to co-author professor Andrew Pollard.
Oxford University has partnered with AstraZeneca to produce the vaccine globally, with the pharmaceutical giant already committed to making two billion doses.
The UK has ordered 100 million doses of the vaccine, while a number of other governments around the world, including the US, France and Germany, have entered into supply deals with the company should the candidate prove effective and gain regulatory approval.
AstraZeneca has said it will not seek to profit from the vaccine during the pandemic.
“If our vaccine is effective, it is a promising option as these types of vaccines can be manufactured at large scale,” said Professor Gilbert.
“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.”
Global Justice Now, which campaigns on issues of trade, health care and justice in the developing world, said “AstraZeneca’s claim that it will sell the vaccine at no profit during the pandemic is not good enough.”
Heidi Chow, a senior policy manager, told The Independent: “It needs to fully disclose its costs and prices so that this can be verified. The company has also left the door open to profiteer from this publicly-funded vaccine after the pandemic, when the need for vaccination may still exist.
“This vaccine has been paid for by the public purse and if it ultimately proves to be safe and effective, it should be a global public good and openly licensed.”
AstraZeneca’s vaccine is among the leading candidates currently being developed, with about a dozen others in the early stages of human testing or poised to start, mostly in China, the US and Europe.
The American biotech firm Moderna, based in Massachusetts, was the first to announce that its vaccine candidate had produced an immune response within people.
The company said in May that the levels of antibodies developed by participants in its phase one trial mirrored those seen in patients who have recovered from the virus.
BioNtech and Pfizer have also delivered positive results from early trials into their MRNA-based vaccine.
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.