How could the study’s pancreatic cancer finding happen by chance?

The short version: chance is a real possibility, but the study design itself is the bigger problem.

The South Korean study found a pancreatic cancer hazard ratio of 2.25 with a p-value under 0.001 [1]. That is a strong statistical signal, but it comes from testing 18 cancer types in one retrospective analysis [1]. When you run that many comparisons, some will cross the significance threshold by random chance alone. The study did not report correcting for multiple testing, which is a standard safeguard against exactly this kind of false positive [1].

The more important issue is what the study cannot control for. It used national health insurance claims data, not a randomized trial [1]. People who get vaccinated differ from people who do not in ways that matter for cancer risk: they visit doctors more often, get more screening, and may have different underlying health conditions. The study found increased risks for pancreatic and gastric cancer but also a decrease in leukemia [1]. That pattern—some cancers up, some down—is exactly what you would expect from residual confounding or chance in a multiple-testing setting, not from a coherent biological mechanism.

The one-year follow-up is also far too short for most solid tumors [1]. Pancreatic cancer typically takes years to develop. If the association were real, you would expect to see it only after longer latency. A signal that appears within one year is more likely to reflect detection bias (vaccinated people getting more imaging that finds existing tumors) or chance than a true causal effect.

My call: the pancreatic cancer finding could easily be a chance result from multiple testing, residual confounding, or detection bias in a single observational study. Confidence: moderate that chance is a plausible explanation, but the study's design cannot rule out a real effect either.