A pedestrian walks through a deserted corridor in Chapel Hill, N.C., where bars, restaurants, and other businesses have closed to stop the spread of coronavirus.

In a recent and controversial First Opinion, epidemiologist and statistician John Ioannidis argues that we lack good data on many aspects of the Covid-19 epidemic, and seems to suggest that we should not take drastic actions to curtail the spread of the virus until the data are more certain.

He is absolutely right on the first point. The U.S. has done fewer tests per capita so far than almost any rich country in the world. And many critical details of the epidemiology — including the absolute number of cases, the role of children in transmission, the role of presymptomatic transmission, and the risk of dying from infection with SARS-CoV-2 — remain uncertain.

On the second point, I would say that his article did what contrarian writing should do: started a discussion. We spoke by phone on Tuesday, not long after his article appeared, and found that we had more in common than it appeared when I first read it.

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So without trying to characterize Ioannidis’ view, I will state a strongly held view of my own: We know enough to act; indeed, there is an imperative to act strongly and swiftly. It is true that we can’t be sure either how many infections there have been in any population or the risk of needing intensive care or the case fatality rate. These uncertainties are two sides of the same coin. Nonetheless, two things are clear.

First, the number of severe cases — the product of these two unknowns — becomes fearsome in country after country if the infection is allowed to spread. In Italy, coffins of Covid-19 victims are accumulating in churches that have stopped holding funerals. In Wuhan, at the peak of the epidemic there, critical cases were so numerous that, if scaled up to the size of the U.S. population, they would have filled every intensive care bed in this country.

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That is what happens when a community waits until crisis hits to try to slow transmission. Intensive care demand lags new infections by about three weeks because it takes that long for a newly infected person to get critically ill. So acting before the crisis hits — as was done in some Chinese cities outside Wuhan, and in some of the small towns in Northern Italy — is essential to prevent a health system overload.

Second, if we don’t apply control measures, the number of cases will keep going up exponentially beyond the already fearsome numbers we have seen. Scientists have estimated that the basic reproductive number of this virus is around 2. That means without control, case numbers will double, then quadruple, then be eight times as big, and so on, doubling with each “generation” of cases.

To stop an epidemic like that permanently, nearly half the population must be immune. While the exact number of people infected in each population is unknown, current estimates are that for every symptomatic case there is about one asymptomatic or very mild case.

In populations with good ascertainment of symptomatic cases, the number of infections is perhaps double what is observed (in the U.S., where testing is limited, true cases are a much higher multiple of reported cases). In well-tested countries, we can be nearly certain that no population has reached anywhere near half of its people infected. That means that when each country lets up on control measures, transmission will increase and the number of cases will grow again.

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It is crucial to emphasize that a pandemic like this does not dissipate on its own, as Ioannidis suggested as a possibility. Severe acute respiratory syndrome (SARS) in 2003 was hammered into submission by intense public health measures in many places, which were effective because transmission was mainly from very sick people. Middle East respiratory syndrome (MERS), which emerged in 2012, is a weakly transmissible infection that causes outbreaks in hospitals, but is otherwise much less contagious than Covid-19.

There are two options for Covid-19 at the moment: long-term social distancing or overwhelmed health care systems. That is the depressing conclusion many epidemiologists have been emphasizing for weeks, and which was detailed in an analysis released this week by the Imperial College London.

Ioannidis is right that the prospect of intense social distancing for months or years is one that can hardly be imagined, let alone enacted. The alternative of letting the infection spread uncontrolled is equally unimaginable. We certainly need more data. Even more than that, we need a breakthrough to make effective treatments, vaccines, or other preventive measures available at scale.

Waiting and hoping for a miracle as health systems are overrun by Covid-19 is not an option. For the short term there is no choice but to use the time we are buying with social distancing to mobilize a massive political, economic, and societal effort to find new ways to cope with this virus.

Marc Lipsitch, D.Phil., is professor of epidemiology at the Harvard T.H. Chan School of Public Health and director of Harvard’s Center for Communicable Disease Dynamics.