Hao et al pooled data from 10 heterogeneous randomized trials (n=3,451) comparing probiotics (various preparations) vs. placebo, with the outcome of upper respiratory tract infections.
Those using probiotics had a reduction in number of URTIs and antibiotic prescriptions by ~1/3 to ~1/2. There was significant heterogeneity among the trials, which included very few people over age 65.
Hao Q et al. Probiotics for preventing acute upper respiratory tract infections. Cochrane Database of Systematic Reviews 2011, Issue 9. Art. No.: CD006895.