Suspected cases and confirmed cases began 2 days before the onset of symptoms, or asymptomatic infected person specimens sampling began 2 days before the start of their close contact but did not take effective protection. Epidemiological investigation professionals based on the results of epidemiological investigations, combined with the information provided by the relevant departments of the big data, based on the following principles to determine close contacts:

1. family members living in the same room *** together;

2. direct caregivers or to provide diagnosis and treatment, nursing services;

3. implementation of the aerosol in the same space may produce aerosol diagnostic and treatment activities Health care workers;

4. In offices, workshops, shifts, elevators, cafeterias, classrooms, and other areas of the same all close contact with the personnel;

5. Confined environment **** meals, **** the same entertainment, as well as the provision of catering and recreational services;

6. Visiting the case of the health care workers, family members, or other close contact with the personnel;

7. People who traveled on the same mode of transportation and had close contact with them, including caregivers and companions on the transportation;

8. People who were exposed to the environment and objects contaminated by the case or asymptomatic infected person;

9. People who were assessed by the on-site investigators as meeting the criteria for close contacts.