"We are one of the countries that has tested the highest percentage of our population and the assumption would be that the more you test, the more mild cases you will have among the confirmed cases, and more mild cases in the denominator will impact the estimation of the mortality," said Vestrheim.
He said that widespread testing may also have made it easier to slow the spread of the disease.
"The more you test, the more people with mild symptoms you find, and you can also do contact tracing and quarantine around people with those mild cases," he said.
Some 5208 people have tested positive in Norway, meaning less than 5 per cent of those who have reported symptoms and been tested turned out to have the virus.
Dag Berild, a medical doctor and associate professor at Oslo University Hospital, argued that the low level of antibiotic-resistant bacteria in Norwegian hospitals may also have played a role in the lower mortality rate.
"Many of the influenza pneumonia cases are complicated by bacterial pneumonia, so if that is also the case with coronavirus, then patients in a country with a low resistance rate among bacteria would have a better prognosis than those in Italy, where they have an awful lot of resistant bacteria, particularly in Lombardy," he said.
Vestrheim said that expanding Norway's testing capacity had not been easy.
"There have been shortages of analytics, for equipment to do the extraction before the PCR testing, for the swabs that you use to take the specimens," he said.
"There's also a huge shortage of protective equipment for health personnel."
But it had been helped by the fact that the country has more than 20 public testing labs in hospitals, as well as ones run by private providers.
"The health service is quite decentralised because of the geography, so we have diagnostic labs all across the country and a good collaborative network among the labs," he said.
As well as testing, Norway has for decades had among the highest number of cases of whooping cough in Europe. This, he said, had made it easy for his institute to roll out the tests rapidly across the country.
He said: "Gradually. more and more labs have established good quality assurance."