Presented this way, the data for Italy clearly show that the infection rate is no longer exponential. The straight line is now a slight downward curve indicating that the rate of increase is slowing.

At a quick glance, the rate of spread in the United States looks similar to Italy’s, at least when plotted on a linear scale. But on a logarithmic scale, it is instantly apparent that the number of Americans becoming infected continues to double every three days or so. That indicates that the limited measures taken until recently did not sever social contact enough to slow the spreading. The U.S. curve has even bent upward in the last few days — an even faster exponential growth — perhaps reflecting more widespread testing.

Italy’s experience shows that more drastic containment measures work, so the U.S. curve may start bending downward in the coming days, as measures here go into effect. (John Burn-Murdoch at The Financial Times maintains a log chart for multiple countries.) The lag between the imposition of measures and their impact on the curve could take days to a week or two, because of the incubation time before symptoms arise. If the line does not begin to bend downward, more stringent actions are probably needed.

But when it finally does, it will herald a real change in the direction of the epidemic in the United States.

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