Fifteen randomised controlled trials (2,557 participants) were included in the review. The authors did not report the findings of the quality assessment in the review but mentioned that they grouped studies into double-blind placebo-controlled randomised clinical trials.
PMBL induced a significant reduction of the number of recurrences of respiratory tract infections compared with placebo (mean difference -0.513, 95% CI -0.722 to -0.303; Ι²=67%, evidence of statistical heterogeneity; 15 RCTs). The number needed to treat for one year to avoid one infection was 1.15.
Subgroup analyses performed on adults and children with recurrent respiratory infections other than COPD, chronic bronchitis and tuberculosis found that PMBL significantly reduced the total number of infections in adults (mean difference -0.502, 95% CI -0.824 to -0.181; Ι²=68%, evidence of statistical heterogeneity; seven RCTs) and in children (mean difference -2.204, 95% CI -3.260 to -1.147; Ι²=0%; three RCTs). The finding was statistically significant in favour of PMBL in tuberculosis patients (mean difference -0.502, 95% CI -0.890 to -0.114; Ι²=13%; two RCTs) but not in COPD or chronic bronchitis patients (mean difference -0.404, 95% CI -0.864 to 0.057; Ι²=60% evidence of statistical heterogeneity; three RCTs).
None of the studies recorded or reported side effects of the interventions.