Because Hymenoptera venom hypersensitivity is potentially life threatening, it seems unethical to perform double-blind, placebo-controlled trials. This may explain why we found only two RCTs and few non-randomised controlled studies but many non-randomised non-controlled prospective or retrospective studies evaluating the effect of VIT. In nearly all studies the effect of VIT was evaluated by measuring the recurrence rates of systemic reactions to re-stings in patients with previous systemic events. Patients with a known history of systemic reactions are at risk of potentially life-threatening reactions to re-stings. In those non-randomised non-controlled trials, it is implicitly assumed that the effect of VIT can be evaluated by comparing the recurrence rates during or after VIT with the recurrence rates in observational studies investigating the natural course of Hymenoptera hypersensitivity (see Prognosis). Another implicit assumption is that the risk for recurrence remains unchanged over time. Many of the trials have addressed the question of when to discontinue VIT, but we found no good and reliable evidence.

One of the two randomised placebo-controlled trials included children who were not randomised properly to the treatment and the analysis was not reported separately for the two allocation groups.19 Since baseline data were similar, the pooled results do not seem to be biased.

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