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First Patient Didn’t Start Washington Outbreak, Simulation Finds

  • New analysis shows state’s spread started later than thought
  • Result suggests contact tracing worked for early patients
A vehicle waits at a traffic signal light on a near empty road in Seattle, Washington, on March 18

A vehicle waits at a traffic signal light on a near empty road in Seattle, Washington, on March 18

Photographer: Chona Kasinger/Bloomberg

For months, researchers have believed that the very first known U.S. case of Covid-19 in mid-January may have seeded an initial large outbreak in the Seattle area, spreading undetected for almost six weeks.

However, a new analysis from researchers at the University of Arizona and elsewhere, based on computer simulations of how the virus evolves, indicates that this is most likely incorrect. Instead, the Seattle-area outbreak likely started weeks later with someone who arrived from Asia around Feb. 13, during a period in which thousands of Americans were returning from China as the outbreak there expanded.