SARS Transmission, Risk Factors, and Prevention in Hong Kong Joseph T.F. Lau,* Hiyi Tsui,* Mason Lau,* and Xilin Yang* We analyzed information obtained from 1,192 patients with probable severe acute respiratory syndrome (SARS) reported in Hong Kong. Among them, 26.6% were hospital workers, 16.1% were members of the same house- hold as SARS patients and had probable secondary infec- tions, 14.3% were Amoy Gardens residents, 4.9% were inpatients, and 9.9% were contacts of SARS patients who were not family members. The remaining 347 case-patients (29.1%) had undefined sources of infection. Excluding those <16 years of age, 330 patients with cases from “undefined” sources were used in a 1:2 matched case-con- trol study. Multivariate analysis of this case-control study showed that having visited mainland China, hospitals, or the Amoy Gardens were risk factors (odds ratio [OR] 1.95 to 7.63). In addition, frequent mask use in public venues, frequent hand washing, and disinfecting the living quarters were significant protective factors (OR 0.36 to 0.58). In Hong Kong, therefore, community-acquired infection did not make up most transmissions, and public health meas- ures have contributed substantially to the control of the SARS epidemic. Acases were reported in Hong Kong (1). Some of the s of June 11, 2003, a total of 1,755 probable SARS sources of SARS transmission are unknown. For instance, the first major SARS outbreak occurred in the Prince of Wales Hospital in March 2003, and 138 probable cases were reported from March 11 to March 25, 2003 (2). Another major outbreak occurred in the Amoy Gardens housing estate on approximately March 26, 2003, and a total of 321 residents were affected (3). A total of 381 hos- pital workers were affected as of May 29, 2003 (4). Other sources of infection are possible. Some inpatients were cross-infected by SARS case-patients, who were hospital- ized for reasons other than SARS; others may have con- tracted the disease through known contacts with other SARS patients. The rest of the community-acquired case- patients contracted the diseases through less defined sources. The distributions of the “known” and “undefined” *Chinese University of Hong Kong, Hong Kong, China sources of infection have not been reported. Such an initia- tive would help assess the infectivity and modes of trans- mission of the virus in the community setting. Also, reports that public health measures, such as wear- ing masks, frequent hand washing, avoidance of crowded places, disinfection of the living quarters had been prac- ticed by most of the Hong Kong population during the SARS outbreak (>90%) (5). The efficacy of widespread use of masks was controversial (6), and evaluating the efficacy of such measures in controlling the epidemic is important. Our study had two objectives. First, we sought to delin- eate the distribution of different sources of transmission of the SARS cases in Hong Kong. The number of cases with known and undefined sources was determined. Patients with known sources included those who were hospital workers, those who lived in the Amoy Gardens Estate, those who were probable secondary cases within a house- hold (i.e., those with another household member who had SARS with an earlier date of onset), those who were inpa- tients and were cross-infected by other inpatients, and those persons who had contact with another SARS patient (who was not one of their household members) before the onset of fever. For the remaining cases, the virus was con- tracted through undefined sources. The second objective was to identify the undefined source group. A number of hypotheses were tested to iden- tify relevant risk and protective factors associated with contracting the disease. Risk factors were related to visit- ing places of potentially high risk and meeting at-risk peo- ple. Preventive factors were related to public health measures for prevention. Methods The study population comprised all probable SARS patients whose cases were reported to the Department of Health on or before May 16, 2003 (n = 1,690). The SARS case definition criteria, used by Hong Kong Hospital Authority, is as follows: radiographic evidence of infiltrates consistent with pneumonia and current temperature >38°C or a history of such at any time in the preceding 2 days, and Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 4, April 2004 587