Norway said Covid-19 vaccines may be too risky for the very old and terminally ill, the most cautious statement yet from a European health authority as countries assess the real-world side effects of the first shots to gain approval.

Norwegian officials said 23 people had died in the country a short time after receiving their first dose of the vaccine. Of those deaths, 13 have been autopsied, with the results suggesting that common side effects may have contributed to severe reactions in frail, elderly people, according to the Norwegian Medicines Agency.

“For those with the most severe frailty, even relatively mild vaccine side effects can have serious consequences,” the Norwegian Institute of Public Health said. “For those who have a very short remaining life span anyway, the benefit of the vaccine may be marginal or irrelevant.”
Pfizer and BioNTech are working with the Norwegian regulator to investigate the deaths in Norway, Pfizer said in an e-mailed statement. The agency found that “the number of incidents so far is not alarming, and in line with expectations,” Pfizer said.

Allergic reactions have been uncommon so far. In the U.S., authorities reported 21 cases of severe allergic reactions from Dec. 14-23 after administration of about 1.9 million initial doses of the vaccine developed by Pfizer Inc. and BioNTech SE.

Norway has given at least one dose to about 33,000 people, focusing on those considered to be most at risk if they contract the virus, including the elderly.