On December 29, 2019, a hospital in Wuhan admitted four individuals with pneumonia and recognized that all four had worked in the Huanan Seafood Wholesale Market, which sells live poultry, aquatic products, and several kinds of wild animals to the public. The hospital reported this occurrence to the local center for disease control (CDC), which lead Wuhan CDC staff to initiate a field investigation with a retrospective search for pneumonia patients potentially linked to the market. The investigators found additional patients linked to the market, and on December 30, health authorities from Hubei Province reported this cluster to China CDC. The following day, China CDC sent experts to Wuhan to support the investigation and control effort. Samples from these patients were obtained for laboratory analyses.

To identify potential cases, a temporary surveillance case definition for the VPUE was established at that time that was consistent with previous “pneumonia of unknown etiology” definitions: an illness of unknown etiology with 1) fever with or without a recorded temperature; 2) radiographic evidence of pneumonia; 3) low or normal leukocyte count or low lymphocyte count during the early stage of disease; and 4) no improvement or worsening symptoms after 3 to 5 days of antimicrobial treatment per standard clinical guidelines (3).

A probable case of VPUE was defined as a surveillance VPUE case or an illness of unknown etiology fulfilling the first three surveillance VPUE case criteria with a history of exposure to the Huanan Seafood Wholesale Market in Wuhan or any other VPUE case.

The initial laboratory screening results from patients were negative for 26 common respiratory pathogens, including influenza A and B virus, parainfluenza virus, adenovirus, respiratory syncytial virus, metapneumovirus, rhinovirus, enterovirus, and other common respiratory viruses. On January 3, 2020, the sequence of novel β-genus coronaviruses (2019-nCoV) was determined from specimens collected from patients in Wuhan by scientists of the National Institute of Viral Disease Control and Prevention (IVDC), and three distinct strains have been established (2). On January 7, this novel coronavirus was confirmed to be the pathogenic cause of this VPUE cluster, and the disease has been designated NCIP.