A dosing error during clinical trials led to the Oxford vaccine reaching 90 per cent efficacy by accident, it has emerged. In the spring, scientists were left baffled as to why participants were experiencing much milder side effects than expected. When they checked, they found participants had received just half the dose given to 500 adults in earlier safety trials. Instead of restarting the trial, researchers at Oxford University boosted the initial participants with a full dose while everyone who enrolled later received the full amount. Trial results announced on Monday show that the 'correct' vaccine dose achieved just 62 per cent efficacy while the 'accidental' arm was 90 per cent effective. Sir Menelas Nicolas Pangalos, executive vice president of biopharmaceuticals research and development at AstraZeneca, the vaccine manufacturer, said: “The reason we had the half-dose is serendipity. “We went back and checked ... and we found out that they had underpredicted the dose of the vaccine by half.” Once informed, regulators allowed the trial of more than 20,000 volunteers to continue – leading to results which were on Monday evening hailed by Prime Minister Boris Johnson as "incredibly exciting news".