Why is there no round trip for flow modulation?

There is no round trip for flow regulation because the round trip is not within the range of 14 days tracked by flow regulation. Travel tracing method: During epidemiological investigation, migrant workers should communicate with patients face to face to find out where they have been in recent 14 days, what means of transportation they have taken, who they have contacted, family members, friends, classmates, colleagues and workers, and who they have contacted, and make detailed records and inquiries.