On the safety and effectiveness of COVID-19 vaccines, certainties and uncertainties

Abstract

Introduction: The COVID-19 vaccines in use (inactivaded virus, encapsulated m-RNA, non-replicating adenovirus-vectored DNA) were clinically tested in randomized placebocontrolled phase-3 studies. Objective: To address certainties and uncertainties about safety and effectiveness of COVID-19 vaccines that were approved for use in various countries. Method: The evidence provided by clinical studies on the efficacy and safety of COVID-19 vaccines was critically appraised. Results: COVID-19 vaccines proved to be efficacious and safe in clinical trials. Adverse events were mostly those of minor severity commonly noted with other vaccines such as injection site pain, mild flu-like symptoms, headache and asthenia. Although being very rare, anaphylaxis-like reactions were noted with mRNA vaccines. Uncertainties regarding vaccine effectiveness refer mainly to the (long-term) duration of immunity provided by vaccination, the degree of protection conferred to elderly people, and how effective vaccines are against emerging SARS-CoV-2 variants. There are few uncertainties about vaccine safety including the absence of clinical trial data in pregnant women (and the impact on the unborn child), children and adolescents. Conclusions: Notwithstanding the knowledge gaps about effectiveness and safety of COVID-19 vaccines (to be further addressed by observational studies), there is overwhelming evidence that public health benefits of vaccination by far outweigh any foreseeable risk.