Randomized clinical trial to determine whether a probiotic formulation containing 4 probiotic strains which belong to Lactiplantibacillus plantarum and Pediococcus acidilactici species can boost the immune response to influenza vaccination evaluated at 4 weeks in a population between 50 and 80 years of age. Secondary outcomes comprise evaluation of immune response at 8 weeks after vaccination, percentage of patients with seroconversion, incidence and duration of influenza-like infections and respiratory infections throughout a 4-month period and safety.

Double blind, randomized, placebo-controlled clinical trial to determine whether supplementation with a probiotic formulation containing 4 probiotic strains (L. plantarum CECT 30292, CECT7484, CECT7485 and P. acidilactici CECT7483) can improve immune response to seasonal influenza vaccination. Subjects with age comprised between 50 and 80 years who meet inclusion and exclusion criteria will be randomized 1:1 to one of the following study groups: one will receive probiotic formulation for 28 days (1 capsule/day) and the other will receive placebo, starting the same day of the influenza vaccination. Main study outcome will be immune response at 4 weeks after vaccination, which will be determined by analysis of specific IgG antibodies in serum against influenza A and B. Secondary outcomes comprise serum titers of specific IgG at 8 weeks after vaccination, percentage of patients with seroconversion at 4 and 8 weeks, evaluation of influenza-like infections or respiratory infections throughout a 4-month period and adverse events.

Inclusion Criteria:

• Subjects 50 - 80 years old receiving seasonal Influenza vaccine in season 2021-2022
• Body mass index (BMI) 18.5-34.9 kg/m2
• Willing to provide informed consent
• Able to follow study procedures, according to the criteria of study investigator

Exclusion Criteria:

• Hypersensitivity to any of the components of influenza vaccine
• Previous influenza infection in the same season (from 01/09/2021)
• Having received influenza vaccination in the previous season less than 9 months before study entry (from 01/01/2021)
• Having received another vaccine -different from COVID19 vaccine- up to 1 month before study inclusion or planning to receive it 1 month after study inclusion
• Active infection at the moment of study entry as diagnosed by study physician
• History of severe allergic reactions (food and non-food), including severe asthma.
• Receiving immunosuppressant therapy, displaying neutropenia, lymphopenia, AIDS, immunoglobulin deficiency, active oncological disease.
• Daily ingestion of any food complement or food added with probiotics (including Actimel o similar products) 2 weeks before inclusion in the study
• Acute pancreatitis, short bowel syndrome or active inflammatory bowel disease
• Diagnosed from other pathologies that according to study physician may compromise the immunological function of the individual (such as immunodeficiencies, lupus, multiple sclerosis, among others)
• Having received antibiotic treatment in the previous month

Exclusion criteria during the study:

• Withdrawal from the study as per study investigator criteria, in case that study procedures are not followed by the participant
• In case of adverse event that prevents them from following study procedures or complying with the treatments under study.
• Failure to comply with study procedures: Complete less than 70% of the days in the patient's diary and/or take less than 80% of the days of the study products.
• Receive any of the unauthorized treatments indicated in the selection criteria, with the exception of antibiotics that will be allowed under medical prescription.

Both placebo and active product will be provided in identical packaging.