Territorywide Study of Early Coronavirus Disease Outbreak, Hong Kong, China

C oronavirus disease (COVID-19) refers to a clus- ter of viral pneumonia cases that first occurred in Wuhan, a city in Hubei Province, China, beginning in December 2019.Etiology was unknown during the early stage of the outbreak until a novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was isolated on January 7, 2020, and the genome was sequenced (1).During the initial outbreak, fever was the main symptom of COVID-19, and about one third of patients experienced acute respiratory distress syndrome.Approximately 16% of patients were in severe condition at admission, and the estimated mortality rate was 1.4% (2).Sustained human-to-human transmission was confirmed upon identification of cases clustering among families and transmission from patients to healthcare workers (3,4), which triggered China's urgent public health actions and international concern.As of February 28, 2020, a total of 78,824 COV-ID-19 cases had been diagnosed in mainland China and 2,788 persons had died.The disease had also spread to 50 other countries (5).The World Health Organization declared COVID-19 a pandemic in March 2020.In Hong Kong, the first imported case was identified on January 23, 2020; the case-patient was a resident of mainland China who traveled to Hong Kong from Wuhan through Shenzhen by high-speed rail.The first local case with unknown source (i.e., patient who had no travel record during the 14-day incubation period) was reported on February 4, 2020 (6).By February 28, 2020, a total of 93 COVID-19 cases had been recorded in Hong Kong; >70 (75.3%) of those were local cases and those case-patients' close contacts (6,7).Secondary and tertiary transmissions were observed in some case clusters.Because source of infection is unknown in most index cases in these clusters, hidden transmission chains were believed to be present in the community.