The company added that the efficacy could be improved to 90% without any serious side-effects, presumably with an eye on results previously announced by Pfizer and Moderna for their vaccine candidates, both reportedly higher than 90%.
U.K. Health Secretary Matt Hancock told Sky News on Monday that British regulators will now need to make sure the vaccine is "effective and safe".
A week earlier, Pfizer Inc and Germany's BioNTech SE said their vaccine candidate had demonstrated greater than 90% efficacy that rose to 95% with analysis of full trial data. A second regimen using two full doses one month apart was 62 per cent effective.
AstraZeneca said its vaccine, developed in collaboration with the University of Oxford, was assessed over two different dosing regimens.
"Excitingly, we've found that one of our dosing regimens may be around 90% effective and if this dosing regime is used, more people could be vaccinated with planned vaccine supply", said Andrew Pollard, chief investigator of the Oxford Vaccine Trial at Oxford, in a statement.
Pascal Soriot, CEO of AstraZeneca, said the development marked an "important milestone" in the fight against the global health crisis.
The last phase of clinical trials showed that the jab had 90% efficacy when given as a half dose, followed by a full dose at least one month apart.
India may grant Pune's Serum Institute of India (SII) with the emergency use authorisation of Oxford and AstraZeneca's coronavirus vaccine if the British-Swedish pharmaceutical company gets such an approval from the United Kingdom government, Niti Aayog member Vinod Paul noted on Saturday. "Furthermore, the vaccine's simple supply chain and our no-profit pledge and commitment to Page 2 of 3 broad, equitable and timely access mean it will be affordable and globally available, supplying hundreds of millions of doses on approval".
The FTSE 100 firm said as more data comes in it will conduct more analysis, refining the efficacy reading and establishing the duration of protection.
The phase three trials evaluated efficacy among more than 16,000 volunteers who received the vaccine or placebo 21 days after the first injection, but the analysis is based on only 20 confirmed cases of coronavirus. All results were statistically significant.
Dr Nabarro said the rollout of the vaccine will take months as he urged the public to remain vigilant under the COVID-19 restrictions. No hospitalizations or severe cases of the disease were reported in participants receiving the vaccine.
AstraZeneca is the third major drug company to report late-stage results for its potential COVID-19 vaccine as public health officials around the world anxiously wait for vaccines that will end the pandemic that has killed nearly 1.4 million people.