I'm really not sure what's going on with this study. While 51 vs 74 is a significant difference, that's still a small sample size (although not in the context of vaccination studies). The question I don't understand is why 30? I'd understand 5% or 95%, but 30%? I can only think of two actual explanations - differences in human genetics or differences in viral.
Perhaps only certain strains (approximately 30%) are susceptible or perhaps only 30% of humans have a immunologic profile that is activated by this vaccination. The scary thought is that somehow this study was poorly done.