Modulation of immune responses to COVID-19 vaccination by intervention on the gut microbiota: a randomized controlled trial.

Older people, who often have more fragile health, have been particularly affected by the COVID-19 pandemic. The results of recent studies show that, while vaccines have shown very good short-term efficacy, the protection of older people may be insufficient, 6 months after the 2nd dose. Some countries have started to offer a 3rd dose. We are considering acting on the intestinal flora of older people (which is often unbalanced) in order to increase the effectiveness of vaccination. Indeed, it has been shown that probiotics (which can rebalance the intestinal flora) significantly increase the production of antibodies after vaccination against the influenza virus. Our hypothesis is that taking probiotics one month before and one month after the 3rd dose of COVID vaccine would provide longer-lasting vaccine protection in older people. Our study will include 668 seniors, aged 65 to 89, who have not had COVID-19, who have received 2 doses of the same vaccine and who will accept a 3rd dose of vaccine. All participants will take one capsule/day (probiotics or placebo) for 2 months and, in the middle of this period, they will receive a 3rd dose of vaccine. Participants will have to travel 3 times to the Sherbrooke Clinical Research Center (inclusion visit, vaccination and final visit). On five occasions (inclusion, vaccination 1 month, 3 months and 6 months post-vaccination), they will prick their fingertip and express the drop of blood on blotting paper. They will mail this dried blood sample in an envelope so that the antibodies can be measured in Quebec City. We expect to reduce by 1/3 the number of seniors poorly protected by the 3rd dose of vaccine 6 months after the injection thanks to probiotics. If successful, this approach could quickly be implemented worldwide because probiotics have few side effects and are affordable.