ONLY one participant out of nearly 24,000 given the Oxford-AstraZeneca Covid vaccine experienced a severe reaction thought to be linked to the jag.

According to a peer-reviewed interim analysis published for the first time today, in the prestigious Lancet journal, one clinical trial patient went on to develop an inflammation to the nervous tissue around their spine, but the individual is now either recovered or recovering.

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Professor Andrew Pollard, director of the Oxford Vaccine Group, said: "We have no safety concerns about the vaccine."

The Oxford vaccine is still undergoing review by the Medicine and Healthcare products Regulatory Agency (MHRA) and is different from the Pfizer-BioNTech vaccine currently being rolled out across the UK.

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The Lancet paper shows that there were no severe Covid cases, deaths or hospitalisations in anyone given the Oxford vaccine, and provides detailed data backing up earlier reports that effectiveness ranges from 62 per cent to 90%, depending on the dosing.

A one and a half dose showed stronger efficacy that two full doses.

There was also evidence that the one and a half dose regimen could reduce asymptomatic Covid infections by 59%, compared to just 4% with two full doses.

Asymptomatic infections are a major factor in the spread of the disease.

The researchers said further data would be collected as trials continue, but wrote: "While a vaccine that could prevent Covid-19 would have a substantial public health benefit, prevention of asymptomatic infection could reduce viral transmission and protect those with underlying health conditions who do not respond to vaccination, those who cannot be vaccinated for health reasons, and those who will not or cannot access a vaccine, providing wider benefit for society

Prof Pollard said it was "premature" to draw any firm conclusions in relation to transmission at this stage, but added: "There's potential for something important there."

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Out of 23,745 clinical trial participants in the UK, Brazil and South Africa, the study reports that there were a total of 175 severe adverse events reported among 168 volunteers.

The vast majority of these - 172 - were not linked to either the Covid vaccine or the placebo, which was a saline injection or an existing inoculation against meningitis.

Of the remaining three incidents, one affected a patient given the Oxford vaccine and one was a patient in the control group.

The Covid vaccine patient developed a case of transverse myelitis which researchers say is "possibly related" to the vaccine.

This is a neurological disorder characterised by inflammation around a section of the spinal cord, damaging the fatty myelin coating that insulates nerve cell fibres.

The control group patient developed haemolytic anaemia, a disorder in which red blood cells are destroyed faster than they can be made.

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The third patient is still 'masked' as part of an ongoing trial in South Africa, meaning it cannot be reported yet whether they received the Covid or placebo vaccine.

This individual developed a severe fever in excess of 40°C, but recovered rapidly and was not hospitalised.

All three volunteers "have recovered or are recovering, and continue to be part of the trial", say researchers.

Two additional cases of transverse myelitis occurred 10 and 68 days following immunisation with the Oxford vaccine.

However, an investigation by an independent committee of neurological experts found that the 10-day patient in fact had pre-existing - but undiagnosed - multiple sclerosis.

They concluded that the second case was also "unlikely" to have been caused by the vaccine, although no further details were given.

A total of four non-Covid deaths were reported across the trial - three in the placebo arm and one in the Oxford vaccine arm.

The causes of death were a road traffic accident, blunt force trauma, homicide, and fungal pneumonia.

Cases of severe Covid disease and hospitalisation were monitored for all 23,745 participants.

From 21 days after the first dose there were 10 cases hospitalised for Covid-19 - all in the control arm. Two were classified as severe, including one death.

The study provided more detail on the demographics of participants.

Of the 23,745 volunteers, 82% were aged 18 to 55, with around 4,100 aged 56 or older.

Data for the 11,636 participants who have reached Phase 3 - the final stage of the clinical trial which measures a vaccine's effectiveness in preventing disease, as well as its safety - 83% were white and only 12% were older adults.

The researchers said recruitment of older volunteers - who are more at risk from Covid - did not get underway until later on in the clinical trials.

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Of these 11,636 participants, 8,895 were given full doses of either the Oxford vaccine or the placebo.

Researchers monitored and tested participants who developed possible Covid symptoms 14 days or later following their second dose.

In this group, 27 out of the 4,440 given the Oxford vaccine went on to contract Covid naturally from the environment, compared to 71 out of the 4,455 given the placebo. This equates to an efficacy of 62% in preventing Covid disease.

At the same time, a subset of 2,741 volunteers were given a half dose followed by a full dose.

In this group, there were three symptomatic Covid infections out of 1,367 given the Oxford vaccine, and 30 out of 1,374 given the placebo - translating to an efficacy of 90%.

However, the one and a half dose group did not include any adults over the age of 55.

It will be up to the MHRA and the Joint Committee of Vaccination and Immunisation what dose regimen to recommend if the Oxford vaccine is approved for use in the UK.

The trial also measured protection against asymptomatic infection by asking 6,638 UK participants to complete weekly Covid tests.

This identified 69 cases in in total of people who tested positive for Covid, but had no symptoms.

In the one and a half dose group, there were seven cases out of 1,120 people given the Oxford vaccine, compared to 17 out of 1,127 in the control group. This works out at an efficacy of 59%.

In the two full dose group there were 22 and 23 asymptomatic infections in the vaccine and control group respectively.

It remains unclear why the lower dose seems to work better, but investigations are ongoing.