China's aviation authority used the circuit breaker policy to cancel flights by American, Delta and United airlines when passengers who tested negative for Covid before takeoff later tested positive once arriving in China. The US Department of Transportation wrote in its order Friday that "actions impairing the operations of Delta, American and United as described above are adverse to the public interest and warrant proportionate remedial action by the department." "US carriers, who are following all relevant Chinese regulations with respect to pre-departure and in-flight protocols, should not be penalized if passengers, post-arrival, later test positive for COVID-19," it said. The 44 flights are operated by Air China, China Eastern Airlines, China Southern Airlines and Xiamen Airlines and are scheduled for departure between January 30 and March 29. The move comes less than three weeks before Beijing hosts the Winter Olympics.