Toxicological Vulnerabilities and Emerging Safety Signals in Modified mRNA Vaccine Technology: A Narrative Review of the COVID-19 Experience

The rapid deployment in early 2021 of modified mRNA (modRNA) as a vaccine platform for the COVID-19 pandemic sparked a wave of optimism for its potential as a preventive medicine. However, this compacted timeframe may not have fully addressed certain toxicological risks associated with this new technology, which would typically require more extensive preclinical testing. Five years postrollout, an accumulating body of postauthorisation evidence has highlighted safety signals that require closer scrutiny, such as those concerning biodistribution, off-target effects and persistent adverse events. Independent analyses of Phase III trial data and pharmacovigilance reports have raised questions regarding benefit-risk balance and potential long-term biological effects. This narrative review examines the evolution of the toxicological understanding of modified RNA as a vaccine technology from platform design through long-term observational data, identifying areas of uncertainty and vulnerability that may inform future development and regulatory evaluation. The review emphasises that fast-tracked decision-making under exceptional uncertainty requires explicit safeguards to prevent premature certainty and to support learning from emerging evidence over time.