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The Cochrane human papillomavirus (HPV) vaccine review missed nearly half of the eligible trials.
The review was influenced by reporting bias and biased trial designs.
Authors of Cochrane reviews should make every effort to identify all trials and the trials’ limitations.
In May 2018, the Cochrane Collaboration published its review of the human papillomavirus (HPV) vaccines.1 The review primarily assessed the vaccines’ effect on precursors to cervical cancer. Cochrane has high standards for its reviews2; however, there were important limitations in its HPV vaccine review, which we address in this paper.
The Cochrane review missed nearly half of the eligible trials
The Cochrane review conducted trial searches up until June 2017 and included 26 randomised trials with 73 428 women.1 In January 2018, we published an index of the study programmes of the HPV vaccines that included 206 comparative studies.3 As of June 2017, about one-third of the 206 studies were not published and half of the completed studies listed on ClinicalTrials.gov had no results posted.3 Although we sent our index to the Cochrane group handling the Cochrane review, the review stated that, ‘nearly all end-of-study reports have been published in the peer-reviewed literature’. When we applied the Cochrane review’s inclusion criteria to the 206 studies, we identified 46 completed and eligible trials. The number of randomised participants could be assessed for 42 of the 46 trials and was 121 704. With nearly half of the trials and half of the participants missing, the Cochrane authors’ conclusion, ‘that the risk of reporting bias may be small’, was inappropriate. Fifteen of the 20 additional trials were listed on ClinicalTrials.gov; the Cochrane authors would therefore have identified more trials if they had searched ClinicalTrials.gov in more depth and searched additional trial registers (we searched 45 trial registers3).
The Cochrane authors stated that they ‘did not include …
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