Epidemiological investigation. The epidemiological investigations of Covid-19 patients, suspected individuals, and their close contacts were conducted within 24 hours upon receiving the report. The information registered includes demographic data, time of symptom onset, travel history and itinerary, close contacts history, and exposure history to XFDM. To identify all visitors to the XFDM during the outbreak and three weeks before the outbreak, especially those visited contaminated booths, records from epidemiological investigations were retrieved and analyzed.
Sample collection. Oropharyngeal swabs from more than 10 million citizens, 9,492 individuals suspected to be infected with SARS-CoV-2 and all 3,311 employees working at XFDM were collected and stored in viral transportation medium. Peripheral venous blood was collected for antibody detections. A thorough survey of all business sites in XFDM was carried out, and 5,342 environmental samples were collected from the surface in each vending booth (cutting board, utensils, processing bench, washbasin, faucet, sewer, and trash bin), public area (elevator button, stair handrail, door handle, trash bin, drain, sewer, faucets, urinals, and toilet), as well as food, packages, and cold storages. Body swabs and package swabs from the salmon fish sealed in the original packages in the supplier’s cold storage outside XFDM were also collected. The data collection for cases was deemed by the National Health Commission as the contents of public health outbreak investigation.