Aspirin in Primary Prevention

Some questions about the use of aspirin remain to be answered. One of the potentially most important concerns it’s role in primary prevention - that is, protection in subjects who have not already had any thrombotic event. This has yet to be adequately tested.
Three trials have been reported. A non-placebo-controlled trial was conducted in 5,139 healthy British doctors, half of whom were given 500 mg aspirin per day for six years. (41) The results give no evidence of protection.

On the other hand, a placebo-controlled trial in 22,071 American physicans, given 325 mg aspirin, was stopped prematurely when a monitoring committee reported a relative reduction by aspirin of 44% in the incidence of non-fatal MI. (42) Subsequent analyses of the complete results raised doubts however about the dependability of this trial because of a remarkably low mortality and an unexpectedly low MI case-fatality rate. (42) Cardiovascular mortality was only 15% of that expected while the case fatality ratio was only 9.5% against an expected of at least 50%.

Ignoring the uncertainties, these trials taken together, suggest a significant protection from non-fatal infarction by aspirin of 33% (42) and a non-significant reduction of all vascular events of 10%. (34)