Expected impact of a public health intervention in the presence of synergistic risk factors.

OBJECTIVE: Elaborate and test a method to extrapolate the population attributable fraction (benefit of an intervention to reduce the exposure of a given population to a given risk factor) to another population allowing for effects of synergistic factors. STUDY DESIGN AND SETTING: Using data from the Systolic Hypertension in the Elderly Program, the present study investigated the impact of a reduction of blood pressure on the occurrence of stroke accounting for the age of the targeted population. RESULTS: A reduction of blood pressure in populations differing by their age distributions showed that the preventable proportion of strokes increased with age. A 20-mmHg reduction of blood pressure in a population with mean age 60 years was associated with a 14% reduction of strokes and 18% in a population with mean age 70 years. The difference between these two proportions can be interpreted as the proportion of cases due to the synergistic actions of age and high blood pressure on the occurrence of stroke. CONCLUSION: The presented example illustrates how the method may be used by public health practitioners to transpose the potential benefits of interventions estimated in a study population to other populations with different exposures to synergistic risk factors.