COPENHAGEN, Jan. 15 (Xinhua) -- An adverse reaction to the first dose of the COVID-19 vaccine may be responsible for as many as 23 reported deaths among frail and elderly patients in nursing homes in Norway, said a press release from the Norwegian Medicines Agency (NoMA) on Friday.

The studies on Comirnaty (Pfizer-BioNTech) included few participants aged over 85 and did not include patients with unstable or acute illness, according to NoMA.

"The assessment suggests that common adverse reactions to mRNA vaccines, such as fever and nausea, may have contributed to a fatal outcome in some frail patients," said Sigurd Hortemo, NoMA's chief physician.

Reports of suspected adverse reactions including death are received on a daily basis and are continuously assessed by both the Norwegian Medicines Agency and the National Institute of Public Health (NIPH), said the press release.

As a result of 13 autopsies of the deceased undertaken for the assessment, the NIPH has updated the COVID-19 vaccination guidelines, providing more detailed advice on the vaccination of the elderly who are frail.

"For those with the most serious frailty, however, even relatively mild vaccine side effects can have serious consequences. For those who have a very short remaining life span anyway, the benefit of the vaccine can be marginal or irrelevant," said the updated NIPH guidelines.

"In Norwegian nursing homes, about 400 people die a week. All deaths that occur in time in connection with the vaccination are carefully assessed, and there is no indication that the vaccine causes deaths," said Steinar Madsen, medical director of the NoMA.

"Doctors must now carefully consider who should be vaccinated. Those who are very frail and at the very end of life can be vaccinated after an individual assessment," Madsen added.

More than 25,000 Norwegians have been vaccinated against COVID-19 since Dec. 27, 2020.