In a controlled exposure study, two cohorts each of 100 volunteers were exposed to different doses of a respiratory virus. Cohort A (low dose) had 12 infections; Cohort B (high dose) had 72 infections. Calculate the fold-increase in infection probability from low to high dose and interpret what this implies about dose-dependence.

• A. 12-fold increase; there is a twelve-times higher risk in the high-dose group because high exposures always double the effect size reported in small cohorts, which is a standard epidemiologic rule.
• B. 1.5-fold increase; the infection probability increased by half again in the high-dose group, implying a modest dose-response effect not worthy of public health concern.
• C. 6-fold increase; the high-dose cohort had a six-times higher infection probability, indicating a substantial dose-dependent effect.
• D. 0.6-fold decrease; the high-dose cohort actually shows a 40% reduction in infection probability relative to the low-dose cohort, suggesting less susceptibility at higher doses.

Answer: (C)