Europe's drug regulator on Thursday recommended adding a rare type of spinal inflammation called transverse myelitis as a side-effect of Johnson & Johnson's single-dose COVID-19 vaccine.

Reports of this serious neurological illness was also at the heart of trial halts in the early stages of development for both AstraZeneca and Johnson & Johnson's shots, which are based on similar technology.

Giving updates on the safety of all coronavirus shots, the European Medicines Agency (EMA) said it was assessing reports of a rare blood condition known as capillary leak syndrome (CLS) following inoculation with Moderna's vaccine.

The EMA said it had recorded six cases of CLS and was assessing all data, but it was not yet clear if there was a causal association between the reports and the vaccine.

In CLS, fluids leak from the smallest blood vessels causing swelling and a drop in blood pressure. The condition has also been studied with vaccines from AstraZeneca and Johnson & Johnson.

The EMA said there was currently not enough evidence of a possible link between rare cases of multisystem inflammatory syndrome (MIS) and mRNA-based vaccines from Moderna and Pfizer-BioNTech vaccine.

The regulator is reviewing if approved coronavirus vaccines could cause MIS. The syndrome is a serious but rare condition in which different body parts become inflamed, including the heart, lungs, kidneys, brain, skin, eyes, or gastrointestinal organs.

Johnson & Johnson and Moderna did not immediately respond to requests for comments.