LONDON: Human trials of the Oxford and AstraZeneca coronavirus vaccine have yet to resume in the US over concerns that the jab may cause a neurological condition that affects the spinal cord.

The US trials have been paused on two separate occasions after two British women receiving the experimental vaccine developed a condition causing inflammation of their spinal cords that can, in serious cases, cause paralysis.

The trials resumed quickly after the first pause when it was discovered that one of the British women had multiple sclerosis, a condition that can cause the same neurological reaction in the spine. But the second pause, first reported two weeks ago, is still ongoing in the US.

The second woman was hospitalized and has recovered, but due to stringent regulations in the US the trials have yet to resume. Trials of the same vaccine have since restarted in the UK, Brazil, India and South Africa.

AstraZeneca, a British-Swedish pharmaceuticals giant, said it conducted safety reviews into the vaccine following the two women’s illnesses, and “after independent review, these illnesses were either considered unlikely to be associated with the vaccine or there was insufficient evidence to say for certain that the illnesses were or were not related to the vaccine.”

On Saturday it released further data on its trial protocol to allay safety concerns, but US regulators and experts remain concerned with the inoculation’s safety.

The US Food and Drug Administration, the country’s main drugs regulator, has not commented but has reportedly requested further data on the two adverse reactions.

Mark Slifka, a vaccine expert at Oregon Health and Science University, said: “If there are two cases, then this starts to look like a dangerous pattern. If a third case of neurological disease pops up in the vaccine group, then this vaccine may be done.”

Should the Oxford vaccine be aborted, it would be a major setback for a research project seen as one of the frontrunners in the global vaccine race.

The US and UK have both invested major sums of money into accelerating the vaccine’s development.

In May, the US government provided AstraZeneca with over $1 billion to speed up its American trials, and the UK has invested over £80 million ($103 million) directly into Oxford University’s side of the vaccine research.