Could the Coronavirus Be Weakening as It Spreads?

Comments from two Italian doctors have triggered a wave of expert rebukes, but not everyone thinks that their views are far-fetched


On May 31, the news agency Reuters published an article with an optimistic but incendiary headline: “New coronavirus losing potency, top Italian doctor says.”

The story included comments from hospital leaders in Milan and Genoa, cities in two regions of northern Italy that have been hit hard by Covid-19. The doctors’ comments were pulled from published news reports in the Italian media, and both suggested that the virus is growing weaker. Matteo Bassetti, MD, PhD, is head of the Infectious Diseases Clinic at the San Martino-IST University Hospital and a professor of infectious diseases at the University of Genoa. He is quoted in the Reuters piece as saying that, “The strength the virus had two months ago is not the same strength it has today.”

Reached for comment on Tuesday, Bassetti elaborated on his prior statement for Elemental. “What is happening in our hospitals — at least in the northern part of Italy — the clinical impression is that the disease is now different compared to the disease of three months ago,” he says. “The majority of patients who presented in our emergency rooms or wards during March and April were very sick with acute respiratory distress syndrome, shock, multiple organ failure, and the majority died in the first days after admission. Now in May, we no longer see these types of patients.”

Bassetti completed a post-doctoral fellowship at Yale University. He’s quick to say that he is not basing his views on lab-confirmed findings; his statements are based on his own interactions with patients and his conversations with other doctors. “The clinical impression here is that the virus is different,” he says. “Is this because the virus lost some viral potency? Or because the viral load or total count of the virus is lower? I don’t know.”

Expert speculation that the virus could be weakening is unpopular — and maybe also dangerous. The Reuters piece was republished in the New York Times and elsewhere, and it triggered an immediate wave of backlash. Health officials within Italy and at the World Health Organization refuted the Italian doctors’ comments, and a panel of U.K.-based experts said that any claims that the virus is weakening are dubious and not supported by evidence.

“I think it’s just not plausible at this point in time,” says Oscar MacLean, PhD, one of those panelists and a bioinformatician at the Institute for Infection, Immunity, and Inflammation at the University of Glasgow in the U.K. “We’ve seen no evidence of widespread attenuation.”

“Yes, the Golden Rule is that viruses tend to mutate and evolve over time to become less pathogenic while we become more resistant, but that doesn’t happen over a matter of a few months — it’s more like a matter of years.”

It’s also worth noting that the push to loosen business and quarantine restrictions in Italy is politicized, as it is in the U.S. Some government officials there have accused their colleagues in the north of manipulating patient data and testing protocols in order to expedite the reopening process in their regions.

But some say the Italian doctors’ claims are plausible — if not proven. “Every time a virus passes from one person to another, it goes through mutations,” says Lee Riley, MD, a professor and chair of the division of infectious disease and vaccinology at the University of California, Berkeley School of Public Health. “Over time these mutations can accumulate, and the virulence of the virus can ultimately lessen.”

He says it’s possible that these mitigating mutations are occurring in Italy — and maybe even in parts of the U.S. “I think this is happening in many places already, including in New York City,” he says. “It’s the nature of these viruses to get tired after a while.”

How some viruses evolve

While plenty of viruses kill, high lethality is not a trait that the most successful viruses possess — at least not in the long term. And no one doubts that Covid-19 could, theoretically, become weaker as time passes. “This is definitely something that can happen,” MacLean says. “Over time there can be a general selection for decreased virulence.”

How does that occur? In oversimplified terms, some different mutations or “strains” of Covid-19 (scientists around the world have already identified roughly 10,000 of them) could be milder than others. And if this mildness helps them spread more rapidly — for example, by helping them elude detection — then that could lead to a softening of the virus over time. But MacLean says that plenty of scientists are on the lookout for these types of changes — in large populations and in small communities — and so far they don’t see evidence that a mild strain or strains are taking hold and displacing more severe ones.

That doesn’t mean patient outcomes are not improving in some places — as Bassetti says he’s observed. MacLean highlights an Italian study published late in May that found that the virus’s lethality in April had “significantly decreased” compared to its lethality in March. But that study’s authors cite improved clinical management as the likeliest cause of the decrease. “This makes sense,” MacLean says. As doctors and health care systems gain experience treating the virus, they’re likely to get better at it. “Along with changes in hospital capacity, triage, and treatment methods, as testing efforts get amplified, more mild and asymptomatic cases will be detected,” he says. “On the ground, that will probably look like a change on the virus’s part, but you need to be far more systematic, and control for those confounders, to make the claim that it’s attenuating.”

Other experts echo his sentiments and say that Covid-19 is unlikely to weaken in the near term. “Yes, the Golden Rule is that viruses tend to mutate and evolve over time to become less pathogenic while we become more resistant,” says Mark Cameron, PhD, an infectious disease researcher and associate professor at Case Western Reserve University in Cleveland. “But that doesn’t happen over a matter of a few months — it’s more like a matter of years.”

Cameron says that coronaviruses, compared to other viruses such as influenza, are genetically very stable, which means they’re less likely to mutate in ways that could lessen the severity of the resulting illness. He says that, in some ways, this stability is a good thing; it means that if and when a Covid-19 vaccine is available, we won’t have to worry about the virus changing in ways that render the vaccine ineffective. “The virus has changed with small mutations since it emerged from Wuhan, however, the sum total of those mutations has not changed the pathogenesis or severity of the disease it causes in humans, or its ability to spread among us,” he explains. “I think it’s highly unlikely that this virus is going to help us out by evolving into a less pathogenic version.”

One of his colleagues at Case Western Reserve, Robert Salata, MD, is a professor of medicine and also physician in chief at the University Hospitals. “To make these statements without genetic evidence, in my opinion, was the wrong thing to do,” he says of the Italian doctors’ comments.

He says that these sorts of unsubstantiated claims are dangerous because they may encourage some people to lower their guard or take fewer precautions, which could lead to an elevated risk of infection. He also says that, in his hospital system in Cleveland, he’s seen no evidence that the virus is weakening or changing.

An uncertain outlook

UC-Berkeley’s Riley says that his belief that Covid-19 could be weakening is not one his colleagues share. “I don’t know of other experts who think the way I do, and I could be totally off the mark,” he allows. “But I look at real-world data instead of predictive models to come up with my ideas.”

He points out that the number of new cases and deaths has steadily declined in New York City and many other hard-hit areas, and that there are no places in Asia where, once the epidemic swept through a city or a community, another wave of similar magnitude returned. He says that there are, of course, multiple explanations for this, and that social-distancing guidelines partly explain the encouraging trends. “But a more likely reason is that the virus itself has evolved during the initial big wave of the epidemic and lost its high pathogenicity,” he says.

He offers a hopeful prediction, though one he readily admits is speculative: “We’ll continue to see low numbers of mild-to-moderate cases, but the severe cases and deaths are likely to gradually disappear over the six months following the initial big wave.” But, he adds, this prediction assumes that most people will continue to wear masks in public places, practice social distancing, and follow other safety measures.

Others say the data — both on Covid-19, and on related viruses — does not support such a sanguine take. “In Toronto in 2003, we thought we’d beaten back SARS, but then we got hit with a second wave that lasted as long as the first,” says Case Western Reserve’s Cameron. “That may be where we are now. We think the worst might be over, but a second wave may wash over us and reset the clock.”

If people underestimate Covid-19 or ignore their state’s public-health guidelines, he says a second wave is still the likeliest scenario. “That’s the risk we’re taking right now if we loosen our public-health response,” he says. “I hope it doesn’t take a new peak in infections to get people to adhere to some of the simple things that they can do to prevent new infections and potential suffering.”

I write about health and science. I live in Detroit with my wife and kids. I’m trying to learn German, but my progress so far is nicht gut.

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