There are limitations (previously discussed here and here) with using 2- or 3-year survival probabilities as primary endpoints. Proportional hazards, while traditionally useful in chemotherapy trials, should indeed be a questionable assumption in immunotherapy research. There are multiple factors to consider when analyzing data from such RCTs, including challenges with causal inferences. In most cases, what’s needed is good statistical modeling using reasonable assumptions. I wish it was always as easy as comparing survival probabilities at fixed time points. Overall survival inferences, for example, are affected by subsequent therapies and this can confound inferences. See here for the related concept of dynamic treatment regimes (DTRs). We are currently analyzing a DTR-based renal cell carcinoma trial where patients were randomized for both first-line and subsequent therapies and even that is not an easy task inferentially. The need for good modeling cannot be overemphasized.