Aspirin given for primary prevention reduced non-fatal MIs but had no significant effect on stroke or cardiovascular deaths, a meta-analysis has shown. Secondary prevention with aspirin, however, significantly reduced the risk of major cardiovascular events.

The Antithrombotic Trialists (ATT) Collaboration is an impressive meta-analysis of 16 secondary and six primary prevention trials of long-term aspirin therapy versus control. This included 17,000 secondary and 95,000 primary prevention patients. There were 3,306 and 3,554 major vascular events (MI, stroke or vascular death) in each respective group.

The results showed that in secondary prevention trials, aspirin significantly reduced the absolute risk of major vascular events (6.7% per year vs 8.2% per year in controls, p

The primary prevention results were less impressive. Aspirin reduced major vascular events by 0.51% per year compared with 0.57% per year in controls, p=0.0001. This was driven by a significant reduction in non-fatal MI 0.18% per year vs 0.23% per year, p

The benefits of aspirin in preventing further events in patients who have already had a cardiovascular event are well established. The magnitude of this benefit tends to offset the risk of adverse outcomes attributed to aspirin such as GI bleeding. In the primary prevention arena the absolute event rate is much lower and therefore the net benefit from interventions is less impressive.

These results would suggest a degree of benefit from the use of aspirin in primary prevention but this is far less clear-cut than in the secondary prevention setting.

Subsequent studies such as the AAA trial have shown even less benefit from aspirin in a primary prevention setting.

However, aspirin continues to be used for the primary prevention of cardiovascular events in those at high risk or with diabetes. Evidence is now gathering that this may not be a sensible policy. Further research is needed to clarify this issue.

In the meantime, I think a practical approach would be to reserve aspirin for the very highest risk primary prevention patients.