3 June 2020 - Americans returning from China landed at U.S. airports by the thousands in early February, potential carriers of a deadly virus who had been diverted to a handful of cities for screening by the Centers for Disease Control and Prevention.

Their arrival prompted a frantic scramble by local and state officials to press the travellers to self-quarantine, and to monitor whether anyone fell ill. It was one of the earliest tests of whether the public health system in the United States could contain the contagion.

But the effort was frustrated as the C.D.C.’s decades-old notification system delivered information collected at the airports that was riddled with duplicative records, bad phone numbers and incomplete addresses. For weeks, officials tried to track passengers using lists sent by the C.D.C., scouring information about each flight in separate spreadsheets.

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