Despite their poor humoral response, FL patients had measurable antigen- specific CD4 + and CD8 + T-cell responses of a magnitude comparable to those of healthy controls [29], suggesting the presence of additional immunological circuitries to control the immune response, including 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. The gene discussed is CD4; the disease is COVID-19.