Monday's statement said "positive high-level results from an interim analysis of clinical trials of AZD1222 in the United Kingdom and Brazil showed the vaccine was highly effective in preventing Covid-19 ... and no hospitalisations or severe cases of the disease were reported in participants".
British Health Secretary Matt Hancock said he felt "a great sense of relief" at the news of the AstraZeneca vaccine's effectiveness.
The pair said that regimen n=8,895 showed 62% efficacy when given as two full doses at least one month apart.
The results are based on interim analysis of trials in the United Kingdom and Brazil of the vaccine developed by Oxford University and manufactured by AstraZeneca. "Excitingly, we've found that one of our dosing regimens may be around 90% effective and if this dosing regimen is used, more people could be vaccinated with planned vaccine supply", Professor Andrew Pollard, Director of the Oxford Vaccine Group and Chief Investigator of the Oxford Vaccine Trial, said in the release.
It added: "One dosing regimen (n=2,741) showed vaccine efficacy of 90 percent when AZD1222 was given as a half dose, followed by a full dose at least one month apart". "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 vaccine can also be administered within existing healthcare settings such as hospitals and doctors.
AstraZeneca chief executive Pascal Soriot said his firm's vaccine would still be highly effective and would have an "immediate impact".
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"Although no serious reactions were reported in people who got the Oxford vaccine, we do need to await the full safety data and to monitor safety of all vaccines carefully if and when they are rolled out".
The data comes after United States rivals published interim data in recent weeks showing efficacy of more than 90%.
"This is important as the vaccine could reduce the spread of the virus as well as protect the vulnerable from severe disease". All results were statistically significant.
And despite a number of candidates - including vaccines from Pfizer/BioNTech and Moderna - showing encouraging results, work on developing other options continues.
Dr Nabarro said the rollout of the vaccine will take months as he urged the public to remain vigilant under the COVID-19 restrictions. Pfizer also announced that its vaccine developed with German partner BioNTech SE was consistent across age and the company did not notice any major side effects in volunteers who participated in the trial.
Russia's Sputnik V vaccine on Nov 11 was also shown to be more than 90% effective, though only based on 20 infections.