Allard R, Boivin J F
Department of Preventive Medicine, Saint-Luc Hospital, Montreal, Quebec, Canada.
Epidemiology. 1993 Nov;4(6):517-23. doi: 10.1097/00001648-199311000-00007.
We considered a simple formulation of the sufficient causes model, in which a preventive agent exerts its effect by preventing a sufficient cause of the disease from occurring, while leaving another sufficient cause unaffected. In a group unexposed to the preventive agent, a case of the disease is caused by whichever of the two sufficient causes occurs alone or first in the subject. Among exposed subjects, the preventive agent prevents only the cases of disease in which the sufficient cause it blocks would have occurred alone, not the cases in which the other sufficient cause also occurs during the study period. The proportion of subjects who would avoid the disease if exposed to the preventive agent is the risk difference. The risk difference varies over time, even when the rates of occurrence of the sufficient causes are constant. It increases to a maximum and then declines, as the subjects who have avoided the disease because of the agent later contract the same disease because of exposure to the other sufficient cause. This maximum and the time at which it occurs are readily computed from the incidence rates of disease among exposed and unexposed subjects.
我们考虑了充分病因模型的一种简单形式，其中预防剂通过阻止疾病的充分病因发生来发挥作用，而另一个充分病因不受影响。在未接触预防剂的人群中，疾病病例是由两个充分病因中单独出现或在个体中首先出现的任何一个病因引起的。在接触预防剂的人群中，预防剂仅预防那些其阻止的充分病因本会单独发生的疾病病例，而不预防在研究期间另一个充分病因也出现的病例。如果接触预防剂就会避免患该疾病的人群比例就是风险差。风险差会随时间变化，即使充分病因的发生率保持不变。它会先增加到最大值然后下降，因为因该预防剂而避免患病的个体后来会因接触另一个充分病因而患上相同疾病。这个最大值及其出现的时间可以根据接触和未接触预防剂人群中的疾病发病率轻松计算出来。
