Background
URTIs include the common cold and inflammation of the trachea and larynx, with symptoms including fever, cough, pain and headaches. Most acute URTIs are caused by viral infections and usually resolve after three to seven days. To reduce the incidence of these infections, specific vaccines are often recommended, especially for children and old people.
Some probiotics (live micro-organisms) can confer a health benefit to the patient when administered in adequate amounts. Lactic acid bacteria and bifidobacteria are the most common types of probiotics. They are commonly consumed in fermented foods, such as yogurt and soy yogurt, or as dietary supplements. However, their effects in preventing URTIs are still poorly understood.
Study characteristics and search date
After searching for all relevant trials in scientific databases, we identified 13 randomised controlled trials (RCTs) published up to July 2014. We could extract and pool data from 12 RCTs, which involved 3720 participants (both genders), including children, adults (aged around 40 years) and older people from Finland, Spain, Sweden, the United States, Croatia, Chile, Thailand and Japan.
Key results
Probiotics were found to be better than placebo in reducing the number of participants experiencing episodes of acute URTI by about 47% and the duration of an episode of acute URTI by about 1.89 days. Probiotics may slightly reduce antibiotic use and cold-related school absence. Side effects of probiotics were minor and gastrointestinal symptoms were the most common.
Quality of the evidence
The quality of the evidence is low or very low mainly due to poorly conducted trials, for example with unclear randomisation method and blinding. Some trials were supported by manufacturers of the tested probiotics and some trials had a very small sample size.
Conclusion
Overall, we found probiotics to be better than placebo in preventing acute URTIs. However, more trials are needed to confirm this conclusion.
Authors' conclusions:
Probiotics were better than placebo in reducing the number of participants experiencing episodes of acute URTI, the mean duration of an episode of acute URTI, antibiotic use and cold-related school absence. This indicates that probiotics may be more beneficial than placebo for preventing acute URTIs. However, the quality of the evidence was low or very low.