AstraZeneca and Oxford University have more work to do to confirm whether their Covid-19 vaccine can be 90 per cent effective, peer-reviewed data published in The Lancet showed on Tuesday.
Once seen as the frontrunner in the development of a vaccine against the coronavirus crisis, the British team were overtaken by US drugmaker Pfizer, whose shots - with a success rate of around 95 per cent - were administered to UK pensioners on Tuesday in a world-first hailed as V-Day.
Detailed results from the AstraZeneca/Oxford trials have been eagerly awaited after some scientists criticised a lack of information in their initial announcement last month.
However, the Lancet study gave few extra clues about why efficacy was 62 per cent for trial participants given two full doses, but 90 per cent for a smaller sub-group given a half, then a full dose.
"(This) will require further research as more data becomes available from the trial," the study said.
Less than six per cent of UK trial participants were given the lower dose regimen and none of them was aged over 55, meaning more research will be needed to investigate the vaccine's efficacy in older people who are particularly susceptible to Covid-19.
Overall efficacy
Pooling the results, overall efficacy was 70.4 per cent, the data on Tuesday showed. That is above the 50 per cent minimum set by the US Food and Drug Administration.
Covid-19 vaccines from Pfizer/BioNTech and Moderna have reported efficacy levels of more than 90 per cent in late stage trials.
However, the AstraZeneca/Oxford vaccine is seen as particularly important to tackling the pandemic in the developing world, as it would be cheaper and easier to distribute.
"The basic message that the overall efficacy across the trials that are reported here is about 70 per cent but with a clear description of its uncertainty," said Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine.
The efficacy could be as low as 55 per cent or as high as 80 per cent
"The statistical uncertainty is that the efficacy could be as low as 55 per cent or as high as 80 per cent. The Pfizer/BioNTech and Moderna vaccines both have efficacies above 90 per cent and are clearly more efficacious under trial conditions."
Asked whether the half, then full dose regimen had been a mistake, Andrew Pollard, director of the Oxford Vaccine Group and chief investigator into the trials, told a news conference it had been "unplanned."
Head of AstraZeneca's non-oncology research and development, Mene Pangalos, said he hoped requests for approval from regulators around the globe could still be submitted this year.
However, Mr Pollard conceded the different results from the two dosage regimens complicated matters.
"The regulators will decide exactly what their label should say," he said, when asked whether regulators might approve the two full shots regiment first, and then potentially the half, then full, shot regiment when more data is in.
Safety first
The Oxford researchers said the vaccine was found to be safe, with three out of the roughly 23,700 participants experiencing serious conditions that were possibly related to either the Covid-19 vaccine or a standard meningitis vaccine given to a control group.
A case of a serious neurological illness, transverse myelitis, was reported 14 days after a Covid-19 vaccine booster shot, which was "possibly related to vaccination," they said.
This case had led to a seven-week trial suspension in the United States and to brief halts in trials in other countries.
A case of fever above 40°C occurred in South Africa, but it was not disclosed whether this participant received the vaccine because the person recovered quickly and is continuing the trial without knowledge of the shot received, as is the norm.
