Gut microbiome manipulation to alter the gut-lung axis may potentially protect humans against respiratory infections. However, clinical trials of synbiotics, one of the microbiota-targeted intervention, in this regard is few. Therefore, this study aims to examine the effect of synbiotics on the incidence and severity of upper respiratory tract infection, gut microbiota composition and function, as well as biomarkers of immune function.

Inclusion Criteria:

• aged 18-65 years；
• good general health as determined by medical questionnaires;
• BMI <35 kg/m2;

Exclusion Criteria:

• known congenital or acquired immune defects；
• allergies and other chronic or acute diseases requiring treatment；
• subjects with chronic gastrointestinal diseases;
• alcohol or drug misuse or both；
• pregnancy or lactation；
• vaccination against influenza within the last 12 months；
• use of oral antibiotics, probiotics, prebiotics, synbiotics, drugs active on gastrointestinal motility, or a laxative of any class within the last month.