Table 1.

Centers for Disease Control and Prevention grading system on quality of evidence and strength of recommendation

Quality of evidence	Strength of recommendation
Evidence from ≥1 properly randomized controlled trial	Strong evidence for efficacy and substantial clinical benefit; strongly recommended/good evidence to support a recommendation for or against use
Evidence from ≥1 well designed clinical trial, without randomization: from cohort or case–control analytic studies (preferable from >1 center); from multiple time-series studies; or from dramatic results from uncontrolled experiments	Strong or moderate evidence for efficacy, but only limited clinical benefit; generally recommended/moderate evidence to support a recommendation for or against use
Evidence from opinions of respected authorities, based on clinical experience, descriptive studies or reports of expert committees	Insufficient evidence for efficacy; or efficacy does not outweigh possible adverse consequences (e.g., drug toxicity or interactions) or cost of chemoprophylaxis or alternative approaches; optional/poor evidence to support a recommendation Moderate evidence against efficacy or for adverse outcome; generally not recommended Strong evidence against efficacy or of adverse outcome; never recommended