(Reuters) – Europe’s medicines regulator has added an extremely rare nerve-damaging disorder, Guillain-Barré syndrome, as a possible side–effect of AstraZeneca‘s COVID–19 vaccine, regular safety updates from the watchdog showed on Wednesday.

The European Medicines Agency said a causal relationship between GBS and the AstraZeneca shot, known as Vaxzevria, was a “at least a reasonable possibility” after 833 cases of GBS were reported out of 592 million doses of the vaccine given worldwide by July 31.

The EMA categorised the side–effect as “very rare“, the lowest frequency of side–effect category it has, and has emphasised that the benefits of the shot outweigh the risks.

The U.S. Food and Drug Administration has added a warning over Guillain-Barré syndrome as a possible side–effect of Johnson & Johnson’s shot. Both vaccines use viral vector technology, and have also been associated with rare blood clots.

The EMA also tagged some other less severe side–effects to vaccines from Johnson & Johnson, Moderna as well as AstraZeneca‘s shot.