Quality assessment
The quality of the included studies was evaluated independently by two authors using the Newcastle-Ottawa Quality Assessment Scale (NOS). The NOS scale includes three domains, the maximum scores are four points for selection(representativeness of the exposed cohorts, selection of the non-exposed cohorts, ascertainment of exposure, and demonstration of the absence of an outcome of interest at study initiation), and two points for comparability (comparability of cohorts based on the design or analysis), and three for outcome assessment (assessment of outcome, follow-up long enough to produce results and adequacy of cohort follow-up). One point is awarded for each item that is satisfied by the study, except for the second part of the scale (comparability of Cohorts on the Basis of the Design or Analysis), which is capped at two points. The scale is scored out of 9, with a cumulative score of >=6 being a high-quality study and less than 6 stars being a low-quality study. The study was evaluated by two investigators, and a third investigator was consulted if there was disagreement.