Quality appraisal of observational studies (indicators from New-Castle-Ottawa scale).

NR, not reported. aIndicates exposed cohort truly representative. bNon-exposed cohort drawn from the same community. cAscertainment of exposure from a secure record. dOutcome of interest not present at start of study. eCohorts comparable on basis of site and etiology of infection. fCohorts comparable on other factors. gAssessment of outcome of record linkage or independent blind assessment. hFollow-up long enough for outcomes to occur. iComplete accounting for cohorts.