A German research institute conducted this study to investigate the long-term effects of probiotic bacteria consumption on various respiratory tract infections. To this end, 479 healthy adults were supplemented daily with vitamins and minerals, and additionally probiotic bacteria (test group) or without probiotic bacteria (control group). The intake of the probiotic led to a significantly shorter duration of these respiratory infections by almost two days and reduced severity of symptoms. You can find more details in the study.
de Vrese et al.