And it looks like we now have a preliminary number for the 'threshold' - that is, the number of infections beyond which the sailing will have to be terminated immediately.
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"CDC ... may require them to immediately end a simulated voyage and take other actions to protect the health and safety of volunteer passengers and crew if a threshold of COVID-19 cases is met or exceeded during the simulation.
During simulated voyages, this threshold is met when 1.5% of COVID-19 cases is detected in passengers or 1.0% of COVID-19 cases is detected in crew.
This threshold may be modified for future simulated voyages or restricted passenger voyages based on lessons learned from simulated voyages or restricted passenger voyages, the evolution of the pandemic, or other factors."
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So, for a ship with 2.000 guests on board, an outbreak among 30 or more will force the termination of the sailing, The ship will then have to make a beeline to the port with which it has the shoreside agreement and begin medical care/quarantine procedures.