A safety panel of the European drug regulator has recommended adding a rare spinal inflammation called transverse myelitis as a side effect of AstraZeneca's COVID-19 vaccine.
The vaccine has faced several setbacks, including production delays and probes by regulators following rare cases of severe side effects such as blood clots with low platelets, which led to several countries restricting or stopping its use.
The European Medicines Agency's safety committee also reiterated its recommendation of a similar warning to be included for Johnson & Johnson's one-shot vaccine.
Transverse myelitis is characterised by an inflammation of one or both sides of the spinal cord and can cause weakness in the arms or legs, sensory symptoms or problems with bladder or bowel function.
"There is currently no confirmed mechanism that has been verified by which a COVID-19 vaccine could cause the very rare event transverse myelitis," an AstraZeneca spokesperson said.
The EMA's committee, after reviewing data, concluded there was a reasonable possibility of a causal relationship between the two vaccines and transverse myelitis.
However, it added the benefit-risk profile of both vaccines remains unchanged.
AstraZeneca did not immediately respond to a Reuters request for comment.
The regulatory agency did not provide any information on how many such cases have been reported after the vaccination was given but said transverse myelitis has been added as an adverse reaction of unknown frequency to the vaccines' product information.
Reports of the serious neurological illness was also at the heart of trial halts in the early stages of development for both AstraZeneca and J&J's shots, which are based on similar technology.
The regulator also recommended updating the product information for AstraZeneca's vaccine regarding rare blood clotting with a low platelet count following the first shot, to say fewer such side effects were observed after the second dose.
Among 1809 cases of the condition called thrombosis with thrombocytopenia syndrome reported worldwide, it said 1643 were reported after the first dose and 166 after the second.