How an illusion of efficacy can be established for any treatment
In determining the efficacy of a medical intervention (such as a drug or vaccine) to stop a particular disease or virus it is typical to assume that the treatment needs time to work before a person is classified as 'treated'.
For example, a person vaccinated against a virus may be classified as 'unvaccinated' until 2 weeks after getting the vaccination. This simple animation with a hypothetical example shows that, with such a classification, a placebo (i.e. no effect) vaccination can be shown to be highly effective.
See also this article for more context https://www.normanfenton.com/post/more-on-the-illusions-of-vaccine-efficacy. and note that this applies to observational studies rather than randomized controlled trials.
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