The elderly are at high risk of morbidity and mortality from COVID-19. They are particularly targeted by recent data: i) faster decline of the immune response in the elderly and ii) increase of COVID infection 6 months after the 2nd dose of vaccine. In addition, they often have an imbalance of the microbiota that plays a key role in the activation of the immune system. Taking probiotics, which improve the microbiota, could increase the vaccine response of the elderly. Indeed, it has been shown in 2 meta-analyses that probiotics improve immune response after influenza vaccination. The investigators believe that probiotics taken at the time of4t dose (booster dose) of vaccine could improve immune memory: i) humoral (cellular Receptor-Binding Domain antibody) and ii) (B lymphocytes, CD4+ T and CD8+ T), in the medium term and would then allow a greater spacing of the booster doses in the future OBJECTIVE: To reduce by 33% the percentage of elders without RBD antibodies, 6 months after the booster dose of vaccine, by taking 2 strains of probiotics compared to placebo. Secondary objectives: To improve the evolution of RBD Ac by longitudinal analysis at 5 different time points (inclusion, vaccination, 1 month, 3 months, and 6 months post-vaccination).- To compare the evolution of neutralizing Ac and memory B, CD4+ T, and CD8+ T cells, at inclusion and 6 months post-vaccination. METHODOLOGYDesign. A randomized, intention-to-treat, blinded, placebo-controlled trial.Investigation products are: for the intervention group, a capsule with 2 strains of probiotics and for control group or a placebo capsule. Participants will take one capsule per day for 30 days (15 d and 15 d after the booster dose of vaccine). Both vaccines (Pfizer-BioNTech, Moderna) may be used depending on availability and state of knowledge . With 1:1 allocation, via the REDCap application, using permuted blocks and stratified according to randomization.gender, age (65-79 and 80-89 years) and vaccine type for the booster dose. Inclusion criteria: (Pfizer-BioNTech or Moderna) for more than 6 months, volunteers for booster dose of vaccine, telephone or internet access, informed consent, and living \<75 km from CRC (only for participants for blood test visits). Exclusion criteria: patients with allergies (soy...), COVID-19 disease, possibly affected cognitive functions, chronically weakened immune system (AIDS...) or undergoing anti-cancer treatment. Five finger-prick dried blood samples (DBS) are planned (inclusion, vaccination, 1, 3 and 6 months post-vaccination).The DBS will be mailed to Quebec City . A telephone or mail follow up will be realised for each participant. A subgroup of 100 participants willing to travel will be invited to visit us at the inclusion and final visit to do a blood test. Fecal self-sampling (inclusion) for microbiota study will be sent to I Laforest-Lapointe's laboratory. The levels of RBD Ac will be determined by a validated Elisa test (5 times). From the Results Criteria.venous blood samples (inclusion and 6 months post-vaccination), will be studied: i) the pseudotyped viral neutralization and ii) the cellular immune memory in flow cytometry (B, T CD4+ and T CD8+ lymphocytes). With 30% undetectable RBD Ac in the placebo group (6 months post-vaccine) and 20% in the intervention group, are required (15% attrition, 80% power, 5% two-sided test). The 668 subjects primary analysis on the dichotomous criterion (Ac RBD) will use a GMM logistic regression model for longitudinal data with covariate variant/time.PROJECT OUTCOMEImproving vaccination and spacing vaccine doses among the elderly are major public health issues. If successful, probiotics could be used worldwide very quickly. They have few side effects, are available over the counter and are affordable.