• Yuri Yokoyama do Nascimento, Matheus Aparecido de Toledo, Daniel Maringelli Pasqui, Rafael L Pacheco, and Rachel Riera.
• Escola Paulista de Medicina, Universidade Federal de São Paulo (Unifesp), São Paulo, SP, Brazil.
• J Eval Clin Pract. 2024 Dec 1; 30 (8): 156515701565-1570.

ObjectivesTo analyse the general and primary outcome-related characteristics of clinical trials protocols on COVID-19 vaccines.Study Design And SettingA meta-research study. A search for clinical trial protocols on COVID-19 vaccines was conducted on the ClinicalTrials.gov platform. We considered all protocols of comparative trials registered up to October 26, 2021.ResultsTwo hundred and eighty-two trials were analysed. The median expected trial duration was 445 days (interquartile range [IQR] = 225), and the median target sample size was 420 participants (IQR = 1638). A retrospective registry (after the start date) was observed for 42.55% of the trials. Randomization procedures were planned by 84.75% and full-blinding procedures by 34.75% of the 282 trials. Most trials were labelled as active or still recruiting, and 14 trials (5%) were completed. None of the 14 trials labelled as completed on our search date had results available. Industry funding was reported by 198 trials (70.2%). Most studies declared more than one primary outcome, usually a safety or immunogenicity outcome, and 59 studies (20.9%) had at least one primary efficacy outcome. The description of the primary efficacy outcomes was limited in most cases, referred to as a non-specified 'efficacy' outcome (18.6%) or described as 'COVID-19 cases' (32.2%).Conclusionthe primary outcomes of clinical trials on COVID-19 vaccines are poorly described, and the registers provide insufficient information about them. The registry was retrospectively fulfilled for many trials, which may lead to bias and research waste. Outcomes were generically described and did not provide transparent information for replication in practice, further trials or meta-analyses.© 2024 John Wiley & Sons Ltd.

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