1. English or Chinese 1. The study does not focus on risk factors
2. Quantitative studies: prospective cohort studies or retrospective case–control studies 2. Conference abstracts
3. Risk factors must be demographic, comorbid, perioperative and other patientdetermined
factors
3. Not an original study (e.g., editorial or literature
review)
4. The studies had to provide sufficient information to estimate the Odds ratio (OR) and
95 % confidence intervals (CI)
4. Studies in those without general population of
infected and non-infected group
5. The studies were original studies of risk factors that had adequate control groups and
reported the actual numbers of patients
6. The studies had to be independent in order to avoid giving double weight to some
studies