| I |
Evidence from randomized controlled trial(s). |
| II-1 |
Evidence from controlled trial(s) without randomization. |
| II-2 |
Evidence from cohort or case–control analytic studies, preferably from more than one centre or research group using clinical outcome measures of vaccine efficacy. |
| II-3 |
Evidence obtained from multiple time series with or without the intervention. Dramatic results in uncontrolled experiments (such as the results of the introduction of penicillin treatment in the 1940s) could also be regarded as this type of evidence. |
| III |
Opinions of respected authorities, based on clinical experience, descriptive studies and case reports, or reports of expert committees. |
