CNN  — 

Doctors in Norway are investigating the deaths of 23 elderly patients who had received the Pfizer/BioNTech coronavirus vaccine, looking into the possibility that adverse reactions to the shot “may have contributed to a fatal outcome in some frail patients.”

The vaccine’s side-effects are rare and usually mild. But they could include fever and nausea, which could be dangerous in very ill and frail patients.

Following the deaths, the Norwegian Institute of Public Health updated its Covid-19 vaccination guide with more detailed advice on inoculating elderly people who are frail or terminally ill.

The new guidance says doctors should evaluate each individual patient to determine whether the benefits of vaccination outweigh the risks of any potential side effects.

As of last Thursday, 42,000 people in Norway had received their first dose of the vaccine. As is the case in many other countries, the elderly and those in nursing homes with underlying health conditions are first in line to be vaccinated, because they face much higher risk of becoming seriously ill from Covid-19.

“Therefore it is expected that deaths close to the time [of the] vaccination may occur. In Norway, an average of 400 people die each week in nursing homes and long-term care facilities,” the Norwegian Medicines Agency (NOMA) said in a statement.

NOMA noted that all deaths that occur within the first few days of vaccination “are carefully assessed” and submitted to the Norwegian ADR health registry.

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said Monday that the deaths have to be put into context of the population that they occurred in – a nursing home with extremely frail individuals.

Speaking at the Choose Healthy Lives Black Clergy Conclave, Fauci said that his understanding was that the deaths followed the booster dose of the vaccine when there are more non-specific symptoms, such as aches, fever and malaise.

“It is conceivable that when you have a very frail individual, such as many who are in nursing homes, that even that amount of stress to them could put them over the top,” Fauci added.

Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, said that so far there was “no evidence that any link between vaccination and death in these vulnerable patients is a causal one.”

Evans told the UK’s Science Media Centre that when people who are at high risk of death get vaccinated, “there will be a certain number of coincidental deaths that occur shortly after vaccination.”

Regulatory bodies across the world are closely monitoring these fatalities, Evans said, and are able to calculate the “expected number” of deaths within various time periods.

“We do not yet know, but it would seem that the observed numbers of deaths is not notably above the numbers expected,” he said, adding that there was “no need for anxiety, but complacency is equally mistaken.”

Fauci agreed with that assessment, saying that the first question is what was the rate of death in the nursing homes in the individuals who never received the vaccine. “Once you get that, you’ll get the true denominator,” he said.

Of the 23 deaths, 13 have so far been assessed by the Norwegian Medicines Agency and the National Institute of Public Health, the Institute said in a statement on Friday.

Sigurd Hortemo, chief physician at the Norwegian Medicines Agency, said th