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Potential-harm boundary comparing a vaccine regimen versus placebo. For each infection diagnosed within 18 months of randomization from the 7th to the 99th (x) the boundary is reached if at least y of the infections were assigned to the vaccine arm. The red and blue lines illustrate the analyses at the 30th and 60th infection, with stopping boundary ≥ 22 and ≥ 40 of the infections in the vaccine arm, respectively.
