• Mariana Barosa and Vinay Prasad.
• NOVA Medical School, NOVA University of Lisbon, Lisbon, Portugal. Electronic address: mariana.barosa@nms.unl.pt.
• Am. J. Med. 2024 Oct 15.

BackgroundPost-licensure observational studies are the mainstay of vaccine safety evaluation. However, these studies have well-known methodological limitations, rendering them particularly vulnerable to unmeasured confounding. We sought to describe high-impact observational studies of vaccine safety, investigate the authors' attitudes towards their study's findings and limitations, and report on spin practices.MethodsWe conducted a Pubmed systematic review of comparative observational studies of vaccine safety published in the six top medical journals from inception to March 2024.ResultsThirty-seven studies were included, spanning publications from 1995 to 2024. Most studies focused on COVID19 and influenza vaccines (n=11, 30%, and n=10, 27%, respectively). Study designs and methodologies varied. Electronic health records (54%), passive surveillance databases (32%) and national registries (27%) were the most common data sources. Negative control outcomes were used in a single study. Residual confounding was conceded in 54% of studies, and an additional 24% did so implicitly. Spin was noted in 48.6% of the studies. This systematic review found that authors of observational vaccine safety studies in high-impact medical journals often acknowledge residual confounding, but rarely use methods like negative control outcomes to better detect unmeasured confounding. Furthermore, spin is common, occurring in approximately 50% of the studies.ConclusionsAlthough our findings are somewhat limited by subjectivity in study assessments, they suggest that editors and reviewers of high-impact journals should ensure the language used in reporting observational studies accurately reflects the findings and their limitations.Copyright © 2024. Published by Elsevier Inc.

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