RESEARCH Table 2. Preventive measures and risk factors reported by cases and controls Matched univariate Matched multivariate Factors Caseb Controlc p valued OR (95% CI) OR (95% CI) 2.09 (1.33 to 3.27)e % visited mainland China (reference=no) 12.7 6.5 1.95 (1.11 to 3.42) 0.020 8.27 (2.32 to 29.49)e % visited PWH (reference=no) 3.6 0.5 7.07 (1.62 to 30.75) 0.009 % visited other hospitals/clinics (reference=no) 40.7 17.0 3.36 (2.49 to 4.54) 3.70 (2.54 to 5.39) <0.001 9.10 (4.87 to 17.00)e % visited Amoy Gardens (reference=no) 15.5 2.0 7.63 (3.77 to 15.43) <0.001 % visited crowded places frequently 21.9 20.8 1.07 (0.76 to 1.50) NS - - (reference=occasionally/seldom/no) % contacted someone with fever or influenza 9.0 6.4 1.42 (0.87 to 2.32) NS - - (reference=no) % social contact with someone who visited 8.2 5.2 1.66 (0.96 to 2.85) NS - - a patient in a hospital (reference=no) % social contact with medical personnel 7.6 8.6 0.87 (0.52 to 1.44) NS - - (reference=no) % had a SARS case in the housing estate 6.6 8.5 0.76 (0.44 to 1.31) NS - - (reference=no) % disinfected the living quarters thoroughly 46.6 74.5 0.30 (0.23 to 0.39)e 0.41 (0.29 to 0.58) <0.001 (reference=no) Wore a mask in public places frequently 27.9 58.7 0.27 (0.20 to 0.37)e 0.36 (0.25 to 0.52) <0.001 (reference=occasionally /seldom/no) Washed hands 11 or more times per day 18.4 33.7 0.44 (0.31 to 0.63)e 0.58 (0.38 to 0.87) 0.008 (reference=1–10 times/day) aN.S., not significant; OR, odds ratio; CI, confidence interval; PWH, Prince of Wales Hospital; -, not used by the multivariate analyses. The reference time period was the 10 days before the date of the patient’s onset of fever. bn = 330. cn = 660. dp values for multivariate OR. ep < 0.005. oughly (OR = 0.41, p < 0.001) during the reference period were significant protective factors (Table 2). Undefined Cases After removing those case-patients who may have con- tracted SARS after visiting the Amoy Gardens, the Prince of Wales Hospital, other hospitals, or an affected country, including mainland China, Singapore, and Taiwan (212 cases of the 330 cases), 118 cases remained undefined. They were likely to be community-acquired cases of unknown sources of transmission. When univariate and multivariate conditional logistic regression analyses were repeated for the 118 cases with undefined sources (after 212 patients who had visited some particular places that were associated with risk for transmission were removed from the analysis) and their controls (n = 236), similar results were obtained. The three public health variables—frequent- ly wearing a mask in public places (adjusted OR = 0.36, p < 0.001), washing hands >10 times a day (adjusted OR = 0.44, p = 0.008), and disinfecting the living quarters thor- oughly (adjusted OR = 0.36, p < 0.001)—remained signif- icant protective factors. Again, similar to the results of the previous analysis applied to the 330 cases, the other five variables (visiting crowded places, having contact with someone with influenzalike symptoms, having social con- tact with hospital visitors, having social contact with med- ical workers, and living with in the same housing estate as other SARS case-patients) were not significant risk factors. Discussion Of the 1,192 participants in this study, approximately 16.1% had probable secondary or tertiary transmission occurring within the household, 26.6% were hospital workers with nosocomial infections, 14.3% were Amoy Gardens patients, and 4.9% were cross-infected inpatients. In 9.9%, SARS might have been contracted when the par- ticipant came in contact with a SARS patient who was a nonhousehold member, which may have occurred in a hos- pital or community setting. SARS may have developed in 18.7% after they visited Amoy Gardens, hospitals or clin- ics, or affected countries. This computation leaves 9.9% as community-acquired cases of an unknown source. The percentage of patients related to Amoy Gardens (someone who lived there or visited there) is 18.8% (224/1,192). The percentage of patients with a hospital connection (hospital workers, inpatients, and visitors) is 44% (525/1,192). The proportion of unknown community- acquired SARS infection among all SARS cases in this study was considerably lower than the proportion of noso- comial infection, which suggests that preventing hospital outbreaks is essential. Of the 330 undefined transmissions, 44.2% of the trans- missions occurred through hospital visitors. Another study on household transmission also indicated that hospital vis- its were a significant risk factor for predicting household secondary infection (9). Therefore, the severity of future outbreaks, if any, would depend on the ability of the hos- 590 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 4, April 2004