Twenty-five RCTs (n=1,706) were included in the review: 19 were randomised-controlled, double-blind clinical trials and 6 randomised-controlled, open clinical trials.
Adequate allocation concealment was reported in 12 studies. The Jadad quality score ranged from 2 to 5 (12 RCTs scored 5, four scored 4, seven scored 3 and two scored 2).
Asthma improvements in general (7 RCTs, n=876): compared with placebo, SLIT was found to significantly reduce asthma severity (RD -0.27, 95% CI: -0.33, -0.21 and RR 0.48, 95% CI: 0.40, 0.57). There was no evidence of significant statistical heterogeneity. The NNT using immunotherapy to avoid leaving one patient with no improvement or worse symptoms was 3.70 patients.
Adverse effects (20 RCTs, n=1,501): compared with placebo, a significant likelihood of adverse effects with SLIT was found (RD 0.07, 95% CI: 0.04, 0.10 and RR 1.83, 95% CI: 1.40, 2.40). There was no evidence of significant statistical heterogeneity. The NNT using SLIT was 14.28 patients. No severe adverse effects were reported. Reported mild adverse events included local reactions such as pruritis, erythema and oedema, which usually occurred within 30 minutes of treatment and generally resolved spontaneously.
Asthma symptoms (9 RCTs, n=303): a non significant reduction in symptoms was found (SMD -0.38, 95% CI: -0.79, 0.03). However, there was evidence of significant statistical heterogeneity. The sensitivity analysis did not significantly alter the results.
Allergic symptoms in general (10 RCTs, n=360): a significant reduction in allergic symptoms was found (SMD -1.18, 95% CI: -1.93, -0.43). However, there was evidence of significant statistical heterogeneity. The sensitivity analysis did not significantly alter the results.
Composite symptom plus medication scores (7 RCTs, n=724): when analysing outcomes for asthma together with rhinitis and conjunctivitis, a significant difference in favour of SLIT was found (SMD -0.79, 95% CI: -1.34, -0.24). However, there was evidence of significant statistical heterogeneity. Only one study looked at asthma alone, which reported that no significant between group differences were found.
Medication use: a significant reduction in the use of medication for asthma, rhinitis and conjunctivitis was found in favour of SLIT (SMD -0.82, 95% CI: -1.25, -0.39), based on 10 RCTS (n=488). However, no significant difference was found in the use of medication for asthma alone (SMD -0.91, 95% CI: -1.94, 0.12), based on 6 RCTs (n=254). There was evidence of significant statistical heterogeneity for both outcomes.
Respiratory function and bronchial provocation tests: significant improvements in favour of SLIT were found on the respiratory function test FEV1% (SMD 1.48, 95% CI: 0.13, 2.82), based on 4 RCTs (n=144), and on the FEF25-75% (SMD 1.06, 95% CI: 0.40, 1.72), based on 2 studies (n=42). No significant between-group differences were found on any of the other tests in this category.