A leading US epidemiologist has accused one of the doctors on the White House’s coronavirus task force of “false reassurance” after she said a model he helped develop to predict the spread of the virus overstated the number of people likely to develop Covid-19 – when in fact it referred to something more like a best-case scenario.

Marc Lipsitch, a professor of epidemiology at Harvard University, has previously criticised the US government for a “feckless” response that has failed to slow the epidemic’s progress, and called for intense social distancing policies coupled with a “massive expansion” in testing capacity.

At the White House’s latest coronavirus briefing, Dr Birx said that to get to a situation where 20 percent of the US population – around 6 million people – contracted the virus, there would need to be a very large number of asymptomatic people. She said that models that would predict that do not tally with the existing data.

“The predictions of the model don’t match the reality on the ground in China, South Korea or Italy, she explained. “We are five times the size of Italy. If we were Italy and did all those divisions, Italy should have close to 400,000 deaths. They are not close to achieving that.

“There is enough data on the real experience with the coronavirus on the ground to really make these predictions much more sound. When people start talking about 20 per cent of a population getting infected, it’s very scary, but we don’t have data that matches that.”

In response, Professor Lipsitch wrote that “Our modeling (done by @StephenKissler based on work with @ctedijanto and @yhgrad and me) is one of the models she is talking about” – and that on that basis, he found Dr Birx’s explanation misleading.

Among critical problems for the US, he wrote, is that it is unproven whether “US-style social distancing” can bring the transmission rate down, and that the US is “woefully behind” in testing capacity.

“Saying that “facts on the ground” are not consistent with 20% of the population getting infected is really quite deceptive. Likely, no population has 20% yet infected (though we can’t be completely sure until serologic testing is widespread).

“But this virus has shown in countries around the world that it can spread rapidly, and a small problem can become a big problem -- that is how exponential growth works.

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“It is a fundamental scientific error to take the current success of containment in some places as a sign that permanent containment is possible. We should work to make it possible, but 1918 flu and, frankly, the germ theory of disease show that containment is a temporary victory.”

Other experts have also criticised the Trump administration for its relatively optimistic tilt compared to other predictions of what might happen as the virus spreads in the US. After Donald Trump himself began calling for the end of social distancing by Easter, Johns Hopkins public health academic Tom Inglesby made clear that the potential consequences would be catastrophic.

“Anyone advising the end of social distancing now, needs to fully understand what the country will look like if we do that. COVID would spread widely, rapidly, terribly, could kill potentially millions in the year ahead with huge social and economic impact across the country.”

While Dr Birx’s latest account of the spread of coronavirus might be optimistic, other members of the task force have been more qualified in their public pronouncements. Most conspicuous is Dr Anthony Fauci, who has himself contradicted the president’s claims in TV appearances.

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Despite some of the president’s more upbeat messaging, the US is now leading the world in confirmed coronavirus cases, with more than 85,000 positive tests. A new study from the Institute for Health Metrics and Evaluation has now forecast that the epidemic could claim more than 81,000 lives by the summer.

As far as epidemiologists are concerned, findings like these are not cause for optimism. As Mr Lipsitch put it: “The scenario Dr. Birx is ‘assuring’ us about is one in which we somehow escape Italy’s problem of overloaded healthcare system despite the fact that social distancing is not really happening in large parts of the US.