Vials of the AstraZeneca/Oxford vaccine are inspected inside a lab © Francis Mascarenhas/Reuters

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Surprising efficacy findings from late-stage trials of the coronavirus vaccine developed by Oxford university and AstraZeneca have been confirmed by further analysis in the Lancet medical journal.

When the headline results of the trial were released on November 23, independent experts were surprised that some participants who had received a half dose of the vaccine followed by a full dose were better protected against Covid-19 than those who had received two full doses.

But the analysis in the Lancet, the first peer-reviewed publication of data from a phase 3 study of a Covid-19 vaccine, has confirmed these findings.

The vaccine achieved an average efficacy rate of 70 per cent among the 11,636 participants in the phase 3 trials in the UK and Brazil. However, the efficacy rate was as high as 90 per cent in a smaller subgroup of 1,367 people, all under the age of 55, who had accidentally received the lower regimen.

The Lancet paper contains further analysis, carried out at the request of its editors and reviewers, which confirmed that the half-dose/full-dose regimen was more effective, even once the results were adjusted for age.

“The difference between the two groups holds up,” said Oxford’s Andrew Pollard, lead author of the study. “It’s something to do with the half dose priming the immune system more effectively than the full dose.”

Ten volunteers in the trial became seriously ill with Covid-19, some of who required hospitalisation. All of them had received placebo.

There was some evidence that vaccination reduced asymptomatic coronavirus infection as well as disease but “we need more data to be confident about that”, Prof Pollard said.

Although the Pfizer/BioNTech and Moderna vaccines, which use different technology to the Oxford vaccine, have reported efficacy close to 95 per cent in their clinical trials, AstraZeneca and Oxford said people should not think of a competitive race between coronavirus vaccines.

“When people start to compare data, it is really important to bear in mind that we need several vaccines,” said Pascal Soriot, AstraZeneca chief executive. “Pfizer’s, Moderna’s and ours are not sufficient to vaccinate everyone around the world.”

In addition to the efficacy results, the Lancet paper includes safety data from a larger group of 23,745 participants in the UK, Brazil and South Africa. One person who received the vaccine developed transverse myelitis — inflammation along the spinal cord — which might be related to the inoculation and another had a high fever but both are recovering.

Mene Pangalos, executive vice-president for biopharmaceuticals at AstraZeneca, said: “We have an effective vaccine that meets regulatory requirements for approval around the world.”

The company has considered adding a new arm to its separate US clinical trial of the vaccine or conducting a new global study to assess the efficacy of half-dose/full-dose regimen. But no decision has been made, Dr Pangalos said.

Sarah Gilbert, Oxford professor of vaccinology, told the Financial Times that more data would also continue to accumulate without the need to recruit further volunteers, as the researchers continued to monitor the health of existing trial participants. Further evidence about the vaccine’s effect on older people will be particularly important because there is limited data so far for those over-55.

Separate trials were planned to assess the effect of the vaccine on children and pregnant women, who have not been included in studies to date, Prof Gilbert added.

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