Norwegian health specialists are investigating the deaths of 23 people who passed away shortly after receiving the Pfizer COVID-19 vaccine.

The vaccination program began in nursing homes December 27; over 25,000 Norwegians have been vaccinated so far. Of those who have died, Norway’s Trondheim 24 reported that all were “frail, old patients,” some of them more than 90 years old. 

While investigations continue — only 13 have been completed — Norwegian Medicines Agency said “if you are very frail, you probably should not be vaccinated.”