In DepthCoronavirus

With COVID-19, modeling takes on life and death importance

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Science  27 Mar 2020:
Vol. 367, Issue 6485, pp. 1414-1415
DOI: 10.1126/science.367.6485.1414-b

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Dutch models of COVID-19 are designed to help prevent overloading of hospitals and the need to transfer patients.

PHOTO: THOMAS ANGUS/IMPERIAL COLLEGE LONDON

Jacco Wallinga's computer simulations are about to face a high-stakes reality check. Wallinga is a mathematician and the chief epidemic modeler at the National Institute for Public Health and the Environment (RIVM), which is advising the Dutch government on what actions, such as closing schools and businesses, will help control the spread of the novel coronavirus in the country.

The Netherlands has so far chosen a softer set of measures than most Western European countries; it was late to close its schools and restaurants and hasn't ordered a full lockdown. In a 17 March speech, Prime Minister Mark Rutte rejected “working endlessly to contain the virus” and “shutting down the country completely.” Instead, he opted for “controlled spread” of the virus while making sure the health system isn't swamped with COVID-19 patients. He called on the public to respect RIVM's expertise on how to thread that needle. Wallinga's models predict that the number of infected people needing hospitalization, his most important metric, will taper off next week. But if the models are wrong, the demand for intensive care beds could outstrip supply, as it has, tragically, in Italy and Spain.

COVID-19 isn't the first infectious disease scientists have modeled—Ebola and Zika are recent examples—but never has so much depended on their work. Entire cities and countries have been locked down based on hastily done forecasts that often haven't been peer reviewed. “It's a huge responsibility,” says epidemiologist Caitlin Rivers of the Johns Hopkins University Center for Health Security, who co-authored a report about the future of outbreak modeling in the United States that her center released this week.

Just how influential those models are became apparent over the past 2 weeks in the United Kingdom. Based partly on modeling work by a group at Imperial College London, the U.K. government at first implemented fewer measures than many other countries—not unlike the strategy the Netherlands is pursuing. Citywide lockdowns and school closures, as China initially mandated, “would result in a large second epidemic once measures were lifted,” a group of modelers that advises the government concluded in a statement. Less severe controls would still reduce the epidemic's peak and make any rebound less severe, they predicted.

But on 16 March, the Imperial College group published a dramatically revised model that concluded—based on fresh data from the United Kingdom and Italy—that even a reduced peak would fill twice as many intensive care beds as estimated previously, overwhelming capacity. The only choice, they concluded, was to go all out on control measures. At best, strict measures might be periodically eased for short periods, the group said (see graphic, below). The U.K. government shifted course within days and announced a strict lockdown.

It's not that the science behind epidemic modeling is controversial. Wallinga uses a well-established model that divides the Dutch population into four groups, or compartments in the field's lingo: healthy, sick, recovered, or dead. Equations determine how many people move between compartments as weeks and months pass. “The mathematical side is pretty textbook,” he says. But model outcomes vary widely depending on the characteristics of a pathogen and the affected population.

Because the virus that causes COVID-19 is new, modelers need estimates for key model parameters. Wallinga is now confident that the number of new infections caused by each infected person when no control measures are taken—which epidemiologists call R0—is just over two. And he trusts data showing that 3 to 6 days elapse between the moment someone is infected and the time they start to infect others.

From a 2017 survey of the Dutch population, the RIVM team also has good estimates of how many contacts people of different ages have at home, school, work, and during leisure. Wallinga says he's least confident about the susceptibility of each age group to infection and the rate at which people of various ages transmit the virus.

Compartment models assume the population is homogeneously mixed, a reasonable assumption for a small country like the Netherlands. Other modeling groups don't use compartments but simulate the day-to-day interactions of millions of individuals. Such models are better able to depict heterogeneous countries, such as the United States, or all of Europe. The World Health Organization organizes regular calls for COVID-19 modelers to compare strategies and outcomes, Wallinga says: “That's a huge help in reducing discrepancies between the models that policymakers find difficult to handle.”

GRAPHIC: IMPERIAL COLLEGE COVID-19 RESPONSE TEAM, ADAPTED BY C. BICKEL/SCIENCE

In their review of U.S. outbreak modeling, Rivers and her colleagues note that most of the key players are academics with little role in policy. They don't typically “participate in the decision-making processes … they sort of pivot into a new world when an emergency hits,” she says. Rivers argues for the creation of a National Infectious Disease Forecasting Center, akin to the National Weather Service. It would be the primary source of models in a crisis and strengthen outbreak science in “peacetime.”

Policymakers have relied too heavily on COVID-19 models, says Devi Sridhar, a global health expert at the University of Edinburgh. “I'm not really sure whether the theoretical models will play out in real life.” And it's dangerous for politicians to trust models that claim to show how a littlestudied virus can be kept in check, says Harvard University epidemiologist William Hanage. “It's like, you've decided you've got to ride a tiger,” he says, “except you don't know where the tiger is, how big it is, or how many tigers there actually are.”

Models are at their most useful when they identify something that is not obvious, says Adam Kucharski, a modeler at the London School of Hygiene & Tropical Medicine. One valuable function, he says, was to flag that temperature screening at airports will miss most coronavirus-infected people.

There's also a lot that models don't capture. They cannot anticipate, say, an effective antiviral that reduces the need for hospital beds. Nor do most models factor in the anguish of social distancing, or whether the public obeys orders to stay home. In Hong Kong and Singapore, “It's 2 months already [of such measures], and people are really getting very tired,” says University of Hong Kong modeler Gabriel Leung. Recent data suggest the virus may be spreading faster again in both cities, putting them on the brink of a major outbreak, he adds.

Long lockdowns to slow a disease have catastrophic economic impacts and may devastate public health themselves. “It's a three-way tussle,” Leung says, “between protecting health, protecting the economy, and protecting people's well-being and emotional health.”

The economic fallout isn't something epidemic models address, says Ira Longini, a modeler at the University of Florida—but that may have to change. “We should probably hook up with some economic modelers and try to factor that in,” he says.

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