The 60-year-old patient had a low number of blood platelets and clots in small and large vessels, as well as bleeding.
The Danish Medicines Agency said a small number of similar cases were found in the European Medicines Agency’s (EMA) database of drug side effects – including most recently in Norway on Saturday.
“The disease picture is highly unusual and is currently being thoroughly investigated by the EMA,” it said in a statement on Sunday.
The Danish Medicines Agency said the list of symptoms were not usually seen with blood clots.
Torben Bjerregaard Larsen, a professor of thrombosis (blood clots) and drug monitoring at Aalborg University Hospital, told Danish newspaper Politiken that a low platelet count was unusual for people with blood clots.
“We will see if there has been a very strong blood clotting process in the body. It can be provoked by, among other things, severe infections or trauma. Otherwise, it is not something we usually associate with blood clots,” he said.
However he added that it was not yet clear the clotting had any link to the vaccine.
”In the many years I’ve worked with blood clots, it’s not something I’ve associated with vaccines,” he said.
Norway said on Saturday that three people, all under the age of 50, who had received the AstraZeneca vaccine were being treated in hospital for bleeding, blood clots and a low count of blood platelets, which were labeled “unusual symptoms” by health authorities.
Denmark, Norway and Iceland said last week they would halt the introduction of the AstraZeneca vaccine.
The Netherlands also said it had seen 10 cases of noteworthy adverse side effects from the AstraZeneca jab, including cases of possible thrombosis or embolisms – a blocked artery caused by a foreign body, such as a blood clot.
But the Dutch drug watchdog said on Monday that none of the cases also included a lowered number of platelets.
The Dutch government announced shortly before midnight on Sunday that it was halting use of the AstraZeneca vaccine, hours after the Republic of Ireland also paused rollout of that jab.
The UK’s medicine authority, the MHRA, on Sunday to urge people across the UK to continue to get their coronavirus vaccine.
Dr Phil Bryan, vaccines safety lead at the UK’s medicines regulator, said people “should still go and get their Covid-19 vaccine when asked to do so”.
Additional reporting by agencies