Three young people who received the coronavirus vaccine manufactured by AstraZeneca subsequently developed internal blood clots or bleeding in the brain, Norway's public health institute Folkehelseinstituttet said on Saturday.
All three work in health care, and the health agency said it would investigate whether there was a link.
"Something like this is rare, but very serious," Steinar Madsen, head of Norway's medicines agency, told broadcaster NRK.
The news out of Norway is the latest instance of health officials eyeing AstraZeneca's Covid-19 vaccine nervously. Bulgaria, Denmark, Iceland and Norway have already suspended its use temporarily while research is conducted into possible health problems.
Multiple medical agencies, including the World Health Organization, have argued that the vaccine is safe. However, the worries about side effects are only compounding problems for the vaccine's reputation, which has suffered because of supply problems to the European Union.
Bruises, burst blood vessels
Norway also recorded multiple cases of young people who developed bruises or burst blood vessels after receiving the AstraZeneca vaccine, said Sigurd Hortemo of the Folkehelseinstituttet.
That could indicate a reduced number of platelets, leading to internal bleeding.
Health authorities in Norway are now asking people under 50 who received the AstraZeneca shot in the last two weeks to watch out for any side effects of bleeding, and to go to a doctor if necessary.
Norway administered 121,820 AstraZeneca doses before suspending its use on Thursday. The decision was prompted by a case in Denmark, where a person died of a thrombosis shortly after getting inoculated. However, Danish authorities have not been able to show a link.
