Can you boost the flu shot with prebiotics and probiotics?

One possible way to boost the immune response to the flu shot is by supplementing with probiotics or prebiotics. There’s evidence to suggest^[1] that both can positively influence the immune response. It also seems that the way both prebiotics^[2][3] and probiotics^[4] influence the immune system could improve the immune response to vaccination. This implies that supplementation could help make flu shots stick. This hypothesis has been examined in multiple clinical trials. However, some of these trials have come up^[5] positive^[6], while others have found no evidence^[7][8] of an effect.