In a rare piece of good news about Covid-19, a team of infectious disease experts calculates that the fatality rate in people who have symptoms of the disease caused by the new coronavirus is about 1.4%. Although that estimate applies specifically to Wuhan, the Chinese city where the outbreak began, and is based on data from there, it offers a guide to the rest of the world, where many countries might see even lower death rates.

The new figure is significantly below earlier estimates of 2% or 3% and well below the death rate for China based on simply dividing deaths by cases, which yields almost 4%. While it is still higher than the average 0.1% death rate from seasonal flu, it raises hopes that the worst consequence of the coronavirus will be uncommon.

Cutting against that optimism is the expectation that, because no one was immune to the new virus, “the majority of the population will be infected” absent the quick arrival of a vaccine or drastic public health interventions such as closing public places and canceling public events, the scientists conclude in a paper submitted to a journal but not yet peer-reviewed.

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The expectation that a “majority” of a population will become infected reflects a worst-case scenario about who encounters whom, something modelers call “homogeneous mixing.” But even the more realistic assumption that not everyone mixes with everyone else means that “at least a quarter to a half of the population will very likely become infected” absent social distancing measures or a vaccine, conclude Joseph Wu and Kathy Leung of the University of Hong Kong, leaders in the modeling of infectious diseases, and their colleagues.

The Covid-19 pandemic is sparking efforts to calculate how deadly this new disease is. One measure is called a case fatality rate. While the formula is simple, it’s difficult to get a precise answer.

The better news involves fatality rates. To calculate those, the researchers used data from Wuhan, especially the age distribution of 425 early cases and 41 early fatalities there.

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The chance of someone with symptomatic Covid-19 dying varied by age, confirming other studies. For those aged 15 to 44, the fatality rate was 0.5%, though it might have been as low as 0.1% or as high as 1.3%. For people 45 to 64, the fatality rate was also 0.5%, with a possible low of 0.2% and a possible high of 1.1%. For those over 64, it was 2.7%, with a low and high estimate of 1.5% and 4.7%.

The chance of serious illness from coronavirus infection in younger people was so low, the scientists estimate a fatality rate of zero.

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As physicians and researchers have seen since the start of the outbreak, many infected people never become sick. As few as 14% of people in Wuhan with early coronavirus infections were being detected, said epidemiologist Jeffrey Shaman of the Mailman School of Public Health at Columbia University, who led a study published on Monday in Science on undocumented coronavirus infections.

“I think there are many more than the [nearly] 70,000” confirmed Covid-19 cases in Hubei province, Shaman told reporters.

That means the “infection fatality rate” — deaths among people who have the virus but might or might not show disease symptoms — is even lower than Wu and his colleagues calculate.

Despite Wuhan’s heroic efforts to treat patients, the suddenness of the epidemic overwhelmed hospitals there, much as it has in northern Italy. In countries that got more advance warning and prepared better, especially if social distancing succeeds in “flattening the curve” enough to dampen what would otherwise be a tidal wave of Covid-19 cases, the death rate is likely to be lower still.

“Fatality risk estimates may not generalize to those outside of [Wuhan], especially during subsequent phases of the epidemic,” Wu and his colleagues write. “The increasing availability of newer, and potentially better, treatment modalities to more patients would presumably lead to fewer deaths.” That’s true even within China, they find: “To date, the death-to-case ratio in Wuhan has been consistently much higher than that among all the other mainland Chinese cities.”

Lest anyone be tempted to downplay the threat, the scientists caution that Covid-19 is on track to infect millions of people. If social distancing fails to dampen the number of cases at any one time, overwhelming health care systems, the death rate would be higher.