GenVec ( GNVC) shares sank Wednesday despite what the company said was a positive update on its experimental pancreatic cancer drug TNFerade. Patients with pancreatic cancer treated with TNFerade plus a standard of care demonstrated a 25% lower risk of death than similar patients treated with standard of care alone, according to an interim analysis of a phase III study released by GenVec on Wednesday. GenVec executives called these results encouraging and said the TNFerade study will continue. On the surface, the data do look positive. After all, pancreatic cancer is very difficult to treat, so if TNFerade is reducing the risk of death, there's reason for optimism. Yet, GenVec shares sank TK after the company released the new data and held a conference call. Some potential concerns pop up once a deeper look is taken of the interim TNFerade results. The 25% lower risk of death attributed to TNFerade corresponds to a "hazard ratio" of 0.75, according to the GenVec interim analysis. Simply defined, the hazard ratio is an estimate of the treatment effect of TNFerade plus the standard of care compared to the treatment effect of the standard of care alone. In other words, the hazard ratio measures the clinical benefit of TNFerade (in this case, overall survival) across all time points of the study to date -- a very rigorous method to determine whether TNFerade works or not. The problem that GenVec may run into, however, is that the interim hazard ratio of 0.75 is likely to increase as this study matures, with more patients enrolled and more patients dying.