The rezpegaldesleukin trial reports a 61% EASI improvement. The placebo arm improved 31%.
That placebo response is not noise, and it is not unique to this trial — it is a consistent feature of atopic dermatitis studies. Trial participants use emollients more diligently, are seen regularly, are observed, and enter trials during a flare, which means regression to the mean works in the treatment's favour. Half of the headline improvement here happened in people receiving no active drug.
The consequence is practical. A patient told that a drug produces a 61% improvement will expect that improvement, and the honest figure to quote is the 30-point difference — what the drug adds beyond attention, emollients and time. Single-arm evidence and open-label extensions in this condition should be read with the same suspicion.
The habit generalises to any condition that flares and remits: eczema, psoriasis, migraine, inflammatory bowel disease. Wherever the natural history includes spontaneous improvement, the placebo arm is measuring something real, and the treatment effect is only what exceeds it.
- Atopic dermatitis placebo arms routinely improve 25 to 35%
- Quote the between-group difference to patients, not the treatment-arm change
- Trial entry during a flare guarantees regression to the mean
- Open-label extensions carry the whole placebo effect uncontrolled
- Same caution applies to any relapsing-remitting condition
The statistics, in plain English
A placebo response combines three separable things: true placebo effect, regression to the mean from enrolling during a flare, and co-interventions like emollients that both arms receive. Randomisation subtracts all three from the treatment estimate, which is why the between-group difference is the only number that isolates the drug.
Read the rest in the app
You have read your two free briefings this month. The app carries all 27 specialties, every morning, free — and this finding is waiting in it.
Scan to keep reading on your phone. No account needed to start.
Tomorrow morning, before your first patient
One edition a day for dermatology, written by the desk, every claim tied to its paper. Six minutes.
Get the app — free