Close contacts are persons who had close contact with suspected cases, asymptomatic infected persons, or confirmed cases without effective protection, and secondary close contacts are persons who had close contact with close contacts without effective protection. Close contacts are persons who had close contact with suspected and confirmed cases before the onset of symptoms or before the sampling of specimens from asymptomatic infected persons without effective protection. Sub-closely-contacts are only persons who have come into close contact with a closely-contacted person without taking effective protective measures. Close contacts are generally subject to centralized isolation, and the duration of centralized isolation for medical observation of secondary close contacts is determined by the results of nucleic acid testing of close contacts; of course, there are differences in local policies, which should be implemented in accordance with local policies.