Twenty-three people in Norway died after a few days of being vaccinated with the first dose of the Pfizer Covid-19 vaccine. Of that number, 13 are nursing home patients and are over 80 years old.

According to the report of British Medical Journal (BMJ) on Friday, “Doctors in Norway have been asked to conduct more thorough evaluations of very frail elderly patients in line to receive the Pfizer-BioNTech vaccine.”

“It may be a coincidence, but we aren’t sure,” Steinar Madsen, medical director of the Norwegian Medicines Agency (NOMA), told the BMJ. “There is no certain connection between these deaths and the vaccine.”

In a statement, Sigurd Hortemo, chief physician of the Norwegian Medicines Agency, said among the vaccine reactions were fever and nausea that may have contributed to the incident.

More than 30,000 people in Norway have received the first shot of Pfizer or Moderna vaccine since last month.

Norwegian authorities have given priority to the immunization of residents of nursing homes, most of whom are elderly with medical conditions and others terminally ill.