World Health Organisation experts and other scientists on Monday said there was no evidence to support a claim by a high-profile Italian doctor that the coronavirus was losing potency.
Prof Alberto Zangrillo, head of intensive care at Italy's San Raffaele Hospital in Lombardy, which bore the brunt of Italy's epidemic, on Sunday told state TV that the coronavirus "clinically no longer exists".
But WHO epidemiologist Maria Van Kerkhove and other experts said Prof Zangrillo's comments were not supported by scientific evidence.
"In terms of transmissibility, that has not changed. In terms of severity, that has not changed," Ms Van Kerkhove said.
It is not unusual for viruses to mutate and adapt as they spread. The pandemic has so far killed almost 380,000 people and infected more than 6.4 million.
Martin Hibberd, a professor at the London School of Hygiene and Tropical Medicine, said major studies looking at genetic changes in the virus did not support the idea that it was becoming less potent in any way.
"With data from more than 35,000 whole virus genomes, there is currently no evidence that there is any significant difference relating to severity," Dr Hibberd said.
Prof Zangrillo, well known in Italy as the personal doctor of former prime minister Silvio Berlusconi, said his comments were backed up by a study conducted by a fellow scientist, Massimo Clementi, which would be published next week.
"We have never said that the virus has changed," Prof Zangrillo said.
"We said that the interaction between the virus and the host has definitely changed."
He said this could be due to different characteristics of the virus, which he said they had not yet identified, or differences in those infected.
The study by Dr Clementi, who is director of microbiology and virology at San Raffaele, compared virus samples from Covid-19 patients at the Milan hospital in March with samples from patients with the disease in May.
"The result was unambiguous: an extremely significant difference between the viral load of patients admitted in March compared to" those admitted last month, Prof Zangrillo said.
Oscar MacLean. of the University of Glasgow's Centre for Virus Research, said suggestions that the virus was weakening were "not supported by anything in the scientific literature and also seem fairly implausible on genetic grounds".
Experts and representatives of Johns Hopkins University, Wake Forest Baptist Medical Centre, George Washington University and Northwell Health also said they were not aware of evidence suggesting that the virus had changed.
“The suggestion by the Italian doctor is potentially dangerous as it gives false reassurance based on no evidence,” said Leana Wen, public health professor at George Washington University.
“There is no scientific evidence for there having been a change in the coronavirus.
"It’s a highly transmittable and highly contagious disease. We need to be as on guard as ever.”