China locked down Hubei province in late January in a last-ditch attempt to slow the novel coronavirus epidemic. But hospital admissions for Covid-19 continued to rise for another four weeks. At the epidemic’s peak, nearly 20,000 patients were hospitalized, more than 10,000 in severe or critical condition.

There are lessons here for the U.S. A rapidly escalating outbreak in a large American city would severely burden an area’s health system. Americans are doing things to try to avert this outcome: working from home, cutting...