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BACKGROUND: Probiotics can modulate the immune system in healthy individuals and may help reduce symptoms related to respiratory infections.OBJECTIVE: The objective of the study was to investigate the effect of the probiotic strain Lactobacillus paracasei subsp. paracasei, L. casei 431 (Chr. Hansen A/S) (hereafter, L. casei 431) on immune response to influenza vaccination and respiratory symptoms in healthy adults.DESIGN: A randomized double-blind, placebo-controlled trial was conducted in 1104 healthy subjects aged 18-60 y at 2 centers in Germany and Denmark. Subjects were randomly assigned to receive an acidified milk drink containing ≥10(9) colony-forming units of L. casei 431 (n = 553) or placebo (n = 551) for 42 d. After 21 d, subjects received the seasonal influenza vaccination. Primary outcome was seroprotection rate (anti-influenza antibody titers by hemagglutination inhibition) 21 d after vaccination. Other outcomes were seroconversion rate and mean titers, influenza A-specific antibodies and incidence, and duration and severity of upper respiratory symptoms. Antibiotic use and use of health care resources were recorded.RESULTS: There was no effect of L. casei 431 on immune responses to influenza vaccination. Generalized linear mixed modeling showed a shorter duration of upper respiratory symptoms in the probiotic group compared with placebo (mean ± SD: 6.4 ± 6.1 d vs. 7.3 ± 9.7 d, P = 0.0059) in the last 3 wk of the intervention period. No statistically significant differences were found for incidence or severity.CONCLUSIONS: Daily consumption of L. casei 431 resulted in no observable effect on the components of the immune response to influenza vaccination but reduced the duration of upper respiratory symptoms. The trial was registered at www.isrctn.com as ISRCTN08280229.