An early halt (for futility or efficacy) is entirely the decision of the sponsoring Company (unless the therapy's deemed unsafe). When the efficacy data is unblinded at the (new) first interim of 149 events, if the DCVax-L group is experiencing progression of disease much later than the placebo group, a "stopping boundary" will be crossed, meaning that it will be clear the primary endpoint will be met, reaching statistical significance. There will no longer be a reason to continue the trial. It would be unethical to do so.
A very low p-value is required for stopping at a first interim, but some 6 months difference between groups at 149 events would do it. Modeling shows DCVax-L group should exceed this handily.
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