Denmark has suspended use of the Oxford/AstraZeneca vaccine currently been circulated across Nigeria because of concerns surrounding fatal blood clots.

Denmark’s health minister Magnus Heunicke said in a tweet translated from Danish: “The health authorities have, due to precautionary measures, suspended vaccination with AstraZeneca following a signal of a possible serious side effect in the form of fatal blood clots.

“It is currently not possible to conclude whether there is a connection. We act early, it needs to be thoroughly investigated.”

Danish health officials had approved the vaccine’s use on March 5 for people aged over 65, citing evidence from a Scottish study.

Heunicke did not specify where the blood clot concerns had come from. Newsweek has contacted the Danish Health Authority for comment.

On March 7, Austrian authorities suspended one particular batch of AstraZeneca COVID jabs—batch number ABV5300—after one person died and another became ill after receiving doses. The death was due to multiple thrombosis, a formation of blood clots in blood vessels, and the illness was pulmonary embolism.

The European Medicines Agency said on March 10 there was no indication vaccination had caused the conditions.

Denmark has so far administered 759,160 doses of COVID vaccines so far, according to Reuters analysis.