Table 1. Distribution of 1,214 severe acute respiratory syndrome cases covered by the studya %b 61.0 16.1 n 727 192d Known sources Probable secondary/tertiary household infection Hospital care workers 317d 26.6 Amoy Gardens residents 170d 14.3 Inpatients 58d 4.9 Unknown sources Contacted SARS patient(s) within 118 9.9 14 days before onset of fever c Undefined sources 347 29.1 Visited Amoy Gardens 54d 4.5 Visited PWH e 12d 1.0 Visited other hospitals or clinics 142d 11.9 Visited an affected country 45d 3.8 None of the above 124 10.4 aIncomplete information for 22 cases; complete information for 1,192 cases. bCalculated based on complete data. cThese patients did not belong to the categories listed under known sources. dThese categories are not mutually excluded, i.e., a respondent may be exposed to more than one category. e PWH, Prince of Wales Hospital. another household member had SARS but fever onset occurred earlier). All the names were verified as being reported to the SARS registry. Another 317 of 1,192 (26.6%) cases were hospital workers; 170 (14.3%) lived in the Amoy Gardens; 58 (4.9%) were inpatients who had been hospitalized for diseases other than SARS and kept on wards with SARS patients. Most infected inpatients were long-term chronic patients and had been hospitalized for >2 weeks before having SARS symptoms. These patients were likely to have been cross-infected. A total of 727 case-patients belonged in one of the four categories (61% of 1,192 cases). Another 118 (9.9%) had come into contact with a SARS patient within a 10-day period before onset of fever. For 347 (29.1%) participants, the source was undefined; these participants were included in the case-control analysis. After excluding 17 case-patients <16 years of age, 330 participants were included in the case- control study. Univariate Case-Control Analysis Of the 330 patients with an undefined source of infec- tion, 48% were men and 52% were women. The mean age of the patient group was 47.1 years for both the male and female case-patients (standard deviation [SD] 18.8 and 19.9, respectively, p > 0.05, t test). The percentage of par- ticipants in the undefined source group in the three periods of the epidemic (before March 25, 2003, from March 26 through April 10, and after April 10) were 24.2%, 36.1%, and 43.5%, respectively. Members of the patient group were more likely than the control group to have visited mainland China (12.7% vs. 6.5%, p < 0.005). One patient had visited Taiwan, SARS in Hong Kong another patient had visited Singapore, two controls had visited Taiwan, and none of the controls had visited Singapore (Singapore and Taiwan were listed as affected areas during the study period). Similarly, patients were also more likely than controls to have visited the Amoy Gardens (15% vs. 2%, OR = 9.10, p < 0.005) (keeping in mind that those who lived in the Amoy Gardens had already been removed from the analysis); patients were more likely than the controls to have visited the Prince of Wales Hospital (3.6% vs. 0.5%, OR = 8.27, p < 0.005) or other hospitals or clinics (40.7% vs. 17.0%, OR = 3.36, p < 0.005) (Table 2). A total of 212 cases of the undefined source group had visited at least one of the above-men- tioned categories of places. Frequency of visiting crowd- ed places was, however, not significant in the univariate analysis (21.91% vs. 20.8%, OR = 1.07, p > 0.05). Members of the case-patient and control groups were not statistically different in the percentage of having come into contact with someone with influenzalike symptoms (those having made contacts with SARS patients were already removed, 9.0% vs. 6.4%, OR = 1.42, p > 0.05). The two groups were also not different in the likelihood of having social contact with someone who had visited a hos- pital (8.2% vs. 5.2%, OR = 1.66, p > 0.05) or having social contact with medical personnel (7.6% vs. 8.6%, OR = 0.87, p > 0.05). Also patients were not more likely to have a known SARS patient living in the same housing estate, after Amoy Gardens patients had already been removed from the analysis (such data were made available to the public by the government after April 12, 2003) (8). Furthermore, matching for the reference period, mem- bers of the case group were less likely than members of the control group to have frequently worn a face mask in pub- lic venues (27.9% vs. 58.7%, OR = 0.36, p < 0.005), to have been washed their hands >10 times a day (18.4% vs. 33.7% OR = 0.44, p < 0.005), and to have disinfected their living quarters thoroughly (46.6% vs. 74.5%, OR = 0.30, p < 0.005). Multivariate Analysis When all the variables that were significant in the uni- variate analysis were used as input for the multivariate stepwise conditional logistic regression analysis, the results show that among the 330 patients with undefined sources, the following were significant risk factors: having visited mainland China (OR = 1.95, p = 0.020, Table 2), having visited the Amoy Gardens (OR = 7.63, p < 0.001), having visited the Prince of Wales Hospital (OR = 7.07, p = 0.009), and having visited other hospitals or clinics (OR = 3.70, p < 0.001) during the reference period. On the other hand, using a mask frequently in public places (OR = 0.27, p < 0.001), washing one’s hands >10 times a day (OR = 0.58, p = 0.008), and disinfecting the living quarters thor- Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 4, April 2004 589