The intestinal microbiota and improving the efficacy of COVID-19 vaccinations

Graphical abstract
Most COVID-19 cases are mild or asymptomatic and recover well, suggesting that effective immune responses ensue, which successfully eliminate SARS-CoV-2 viruses. However, a small proportion of patients develop severe COVID-19 with pathological immune responses. This indicates that a strong immune system balanced with anti-inflammatory mechanisms is critical for the recovery from SARS-CoV-2 infections. Many vaccines against SARS-CoV-2 have now been developed for eliciting effective immune responses to protect from SARS-CoV-2 infections or reduce the severity of the disease if infected. Although uncommon, serious morbidity and mortality have resulted from both COVID-19 vaccine adverse reactions and lack of efficacy, and further improvement of efficacy and prevention of adverse effects are urgently warranted. Many factors could affect efficacy of these vaccines to achieve optimal immune responses. Dysregulation of the gut microbiota (gut dysbiosis) could be an important risk factor as the gut microbiota is associated with the development and maintenance of an effective immune system response. In this narrative review, we discuss the immune responses to SARS-CoV-2, how COVID-19 vaccines elicit protective immune responses, gut dysbiosis involvement in inefficacy and adverse effects of COVID-19 vaccines and the modulation of the gut microbiota by functional foods to improve COVID-19 vaccine immunisations.