Most of the virus spreading in the United States can be traced to the introductions from Europe. The investigation showed low-level community transmission was happening as early as late January in California and Washington state, after importation of cases from China. That, too, is consistent with what is know about community transmission: It takes time, many weeks, to reach a critical mass that triggers alarms. The CDC report raises intriguing speculation about how the first known case in the United States may have initiated a spread in a fashion so stealthy that it was impossible to discern at the time. At that point, the standard protocol in contact tracing did not extend to potential presymptomatic spread, the CDC said.

Source: Washington Post May 29, 2020 23:03 UTC