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Has the novel coronavirus in Italy changed in some significant way? That was the suggestion of a top doctor in northern Italy who reports that patients to his hospital have been showing up with lower levels of the virus in their upper respiratory tracts compared with those two months ago.

Alberto Zangrillo, head of San Raffaele Hospital in Milan, roiled the global public health community on Sunday when he told RAI, the national TV station, that “the virus clinically no longer exists in Italy,” with patients showing minute amounts of virus in nasal swabs. Zangrillo theorized in a follow-up interview with The Washington Post that something different may be occurring “in the interaction between the virus and the human airway receptors.”

He added, “We cannot demonstrate that the virus has mutated, but we cannot ignore that our clinical findings have dramatically improved.”

The comments, which received widespread attention following a Reuters report, prompted vigorous pushback from Michael Ryan, a top official with the World Health Organization, who said Monday during an online news conference that “we need to be exceptionally careful not to create a sense that all of a sudden the virus by its own volition has now decided to be less pathogenic. That is not the case at all.”

The consensus among other experts interviewed Monday is that the clinical findings in Italy likely do not reflect any change in the virus itself.

Zangrillo’s clinical observations are more likely a reflection of the fact that with the peak of the outbreak long past, there is less virus in circulation, and people may be less likely to be exposed to high doses of it. In addition, only severely sick people were likely to be tested early on, compared with the situation now when even those with mild symptoms are more likely to get swabbed, experts said.

The pandemic is evolving rapidly, with the rate of new cases declining in some hard-hit areas of the world, including northern Italy and New York City, while rising dramatically in Brazil, Peru and India. The virus, however, is mutating at a slow rate, experts say.

Some strains of the virus have become more dominant, but there is no firm evidence yet that any of them are more contagious or deadly, according to scientists who have reviewed recent genetic studies.

Vaughn Cooper, an infectious-disease expert at the University of Pittsburgh School of Medicine, said the new coronavirus mutates slowly compared with influenza and other microbes, and its genetic changes appear to be “mostly inconsequential.”

“I believe it’s safe to say that the differences that doctors are reporting in Italy are entirely due to changes to medical treatment and in human behavior, which limit transmission and numbers of new infections initiated by large inocula — a larger dose of virus appears to be worse — rather than changes in the virus itself,” he said.

All viruses evolve over time, and many infectious-disease experts think the novel coronavirus will eventually become less lethal to human beings, joining four other coronaviruses in causing common colds. But there is no solid evidence so far that it has changed significantly in the five months since it was first recognized among patients in Wuhan, China.

“The virus hasn’t lost function on the time scale of two months,” said Andrew Noymer, an epidemiologist at the University of California at Irvine. “Loss of function is something I expect over a time scale of years.”

In the United States, the pandemic has taken on a patchwork pattern, with much of the Northeast seeing marked improvement. But some places in the South — Alabama, Texas and Virginia, for example — as well as Wisconsin, California and Washington state are showing increases in confirmed cases, according to the coronavirus tracker of Johns Hopkins University.

“Every place has a different epidemic, and it’s not because of the virus,” said Jennifer Nuzzo, an epidemiologist at the Johns Hopkins Center for Health Security.

Slight tweaks in the microbe’s genetic makeup appear in different places on the planet. Epidemiologists use those mutations to track the virus’s spread. Those changes are akin to stickers slapped on a well-traveled suitcase — markers of where the luggage has gone that don’t impart any functional change.

Researchers Harm van Bakel, Emilia Sordillo and Viviana Simon at the Icahn School of Medicine at Mount Sinai, who have been focusing on the genetics of the novel coronavirus, said in an interview that they had not seen a dip in viral load among patients in that hospital system since March, nor have they detected any major genetic changes in the virus in New York City.

People in the United States are collectively holding their breath, meanwhile, to see if there is an uptick in cases in response to the reopening of the economy, public gatherings over the Memorial Day holiday weekend and the eruption of protests against police violence in cities in recent days.

Nuzzo pointed out that it usually takes around five days, and up to 14 days, for an infection to result in symptoms, and then there is a further time lag before someone seeks a test and gets a result. There is also a lag between when symptoms begin and a person with a serious illness requires hospitalization. Thus it may be several weeks before the lessening of social distancing could result in a detectable change in the trajectory of a local epidemic, she said.

Compounding the uncertainty is the lack of a clear understanding of where and how the virus is spreading, because the country hasn’t done the extensive testing and contact tracing to know where or how infections have occurred.

“We never knew where the transmission was occurring in the United States. And still don’t know,” she said. “Is it risky to go to the grocery store?”

Caitlin Rivers, an epidemiologist and senior scholar at Johns Hopkins, noted that it is hard to tell to what degree people are practicing recommended safety measures, such as social distancing, in communities that have eased stay-at-home orders. She is most concerned about crowded institutions in which social distancing is difficult or impossible.

“I think we will continue to see explosive outbreaks connected to institutions,” Rivers said.

Of the 10 counties in the United States experiencing the greatest increase in their seven-day new case average from Friday, May 22, to Friday, May 29, at least nine have experienced outbreaks at a correctional facility, detention center, food processing center, or long-term-care facility.

“An outbreak starts at an institution, then it starts to move into the community,” Rivers said. “We can’t just say, ‘It’s there at that place, it’s irrelevant to the rest of us.’ That’s not true.”