Lau 2004a.

Methods	Case‐control study carried out in Hong Kong, SARS of China during 4 April to 10 June 2003, at the height of the SARS outbreak. The aim was to describe the defined and undefined sources of SARS cases groups and assess the protective effects of various public health measures Defined sources were classified as being a healthcare worker in a hospital, living in Amoy Gardens (a known focus of infection) having had a contact with a member of the household with SARS of earlier onset, hospital in patients infected with SARS by other hospital inpatients and contacts of SARS cases before the onset of their own symptoms The undefined sources group of cases were all the other categories Cases in general were identified and interviewed on the phone. Households with more than 1 index case were considered as having 2 index cases. Of the 1690 identified cases, 1214 from 996 households were enrolled in the study. 140 cases could not be contacted as they had a wrong phone number, 163 were uncontactable after at least 5 attempts, 163 refused to take part and 10 did not speak either Chinese or English. 17 were further excluded because they were aged less than 16. 22 questionnaires were unusable. (This makes 1175, obviously the 17 minors are included in the case‐control study, as adding them makes a total of 1192)
Participants	Description of cases: 330 probable cases of SARS selected as follows. From 1192 people with probable SARS reported to the Department of Health in the territory of HK up to 16 May 2003, 1175 were entered in the case‐control analysis. SARS cases were defined as Xray evidence of pulmonary infiltration consistent with pneumonia with a temperature of > 38°C or a history of such in the previous 2 days and at least 2 of the following: history of chills in the previous 2 days new or increased cough, breathing difficulty, general malaise of myalgia, typical signs of consolidation and known exposure to SARS. The authors say that this definition is the same the WHO's case definition of probable SARS. At interview, risk factors were elicited and identified. There were 727 cases in the defined source category and 347 in the undefined sources category (330 after exclusion of 17 minors) Description of controls: 660 controls of undefined origin and with no description of selection
Interventions	Natural exposure to SARS during a serious epidemic
Outcomes	Community transmission of SARS reduced OR 0.30 (95% CI 0.23 to 0.39)
Notes	Risk of bias: medium (inconsistencies in the text: lack of description of controls) Notes: the authors conclude that community transmission was of less importance than previously thought and public health measures worked. The following risk factors were significantly associated with SARS (matched multivariate analysis OR with 95% CIs): ‐ Visit to mainland China 1.95 (1.11 to 3.42) ‐ Visited Prince of Wales Hospital 7.07 (1.62 to 30.75) ‐ Visited other hospitals 3.70 (2.54 to 5.39) ‐ Visited Amoy Gardens 7.63 (3.77 to 15.43) The following activities/interventions had a significant protective function: ‐ Thorough disinfection of living quarters 0.41 (0.29 to 0.58) ‐ Wore a mask in public places frequently 0.36 (0.25 to 0.52) ‐ Washed hands 11 or more times a day 0.58 (0.38 to 0.87) Potentially a very interesting study possibly rigorously conducted let down by a very confusingly written text. The biggest problem is lack of clarity as to who the controls were. This may be a reflection of the pressure of carrying out a study in the midst of a serious epidemic
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	N/A
Allocation concealment (selection bias)	Unclear risk	N/A
Blinding (performance bias and detection bias) All outcomes	Unclear risk	N/A
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	N/A
Selective reporting (reporting bias)	Unclear risk	N/A