The efficacy of COVID-19 vaccines varies between individuals and populations, and the reasons for this are still not fully understood. Recent clinical studies and animal models have indicated that the gut microbiota may influence the immunogenicity of the vaccine and, thus, its effectiveness. This suggests that there is a bidirectional relationship between the gut microbiota and the COVID-19 vaccine, with the varying components of the microbiota either enhancing or reducing the vaccine's efficacy. To put an end to the spread of the COVID-19 pandemic, the necessity of vaccines that create powerful and long-term immunity is now more important than ever, and understanding the role of the gut microbiota in this process is essential. Conversely, COVID-19 vaccines also have a significant effect on the gut microbiota, decreasing its total number of organisms and the variety of species present. In this Review, we analyze the evidence that suggesting an interaction between the gut microbiota and COVID-19 vaccine effectiveness, consider the immunological mechanisms that may be responsible for this connection, and explore the possibility of using gut microbiota-focused interventions to improve the efficacy of COVID-19 vaccines.