Fourteen studies were included in the review, although three were studies at different follow-up points of the same RCT population, which made the total sample size 4,536 participants (range 30 to 937). All trials scored 2 or 3 on the Jadad scale, with a mean of 2.4. Most studies were not blinded. Duration of follow-up ranged from six months to seven years.
Prevention of asthma: Interventions reduced levels of physician-diagnosed asthma (RR 0.79, 95% CI 0.66 to 0.94, p=0.009; five studies). There was no evidence of statistical heterogeneity. There were no significant changes in the levels of patient-reported wheeze (five studies). Significant heterogeneity was seen (p=0.005).
Improvement in diagnosed asthma: Intervention reduced the number of days ill with asthma (effect size -0.36, 95% CI -0.59 to -0.13, 2 studies) but did not improve lung function (three studies).