The huge success rate of the AstraZeneca-Oxford University coronavirus vaccine was achieved due to a dosing error, it has emerged.

Yesterday scientists working on the project said the jab had an overall efficacy of 70%, but that this rose to 90% in volunteers given an initial half dose, followed by a full dose.

At first it wasn’t clear why this was the case, but AstraZeneca’s head of biopharmaceutical research Mene Pangalos has since said it was down to a lucky accident.

The plan was for trial participants the UK to receive two full doses, but researchers were perplexed when they noticed that side effects, such as fatigue, headaches or arm aches were milder than expected.

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Dr Pangalos 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.’

After realising their mistake, the team decided to press ahead with that group of volunteers and administered the second, full dose shot, at the scheduled time.

The results showed the vaccine was 90% effective among this group of 3,000 people.

A larger group who had received two full doses produced an efficacy read-out of 62%, leading to an overall efficacy of 70% across both dosing patterns, Dr Pangalos said.

He added: ‘That, in essence, is how we stumbled upon doing half dose-full dose (group). Yes, it was a mistake.’

Scientists said the half dose could help prepare the immune system with a smaller sample of Covid-19 before triggering a full-blown immune reaction with a full dose.

Director of the Oxford Vaccine Group and chief investigator of the trial, Professor Andrew Pollard said: ‘What we don’t know at this moment is whether that difference is in the quality or the quantity of immune response.

‘And that’s something we’re going to be digging into over the next weeks.’

Doctors are likely to be advised to use the 1.5 dose regimen instead of two full doses as it appears to be more effective.

At an estimated £2 per dose, this also means more people can be vaccinated for less money.

For example, the 100million doses pre-ordered by the UK could now cover 66million people instead of the intended 50million.

Oxford researchers say they plan to publish their findings in full in the coming weeks before submitting an application to the UK’s Medicines and Healthcare products Regulatory Agency.

If approved by the regulators, the jab could be administered as early as next month.

AstraZeneca UK president Tom Keith-Roach said 19million doses could be ready by the end of 2020, including the four million already on standby.

He added: ‘This is clearly not the end of the war, but it’s a bit like the discovery of radar in the Second World War to me. 

‘Now we actually have the technology that we need to fight this virus and win.’

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