Sometimes patients are put on aspirin monotherapy, but studies show that it’s not as effective as coumadin or the NOACs. Aspirin by itself reduces the risk of stroke by 20%, but it’s not a significant difference from placebo. The Stroke Prevention in Atrial Fibrillation II provides evidence for this.
As a tangent, the ACTIVE trials looked at aspirin monotherapy vs dual antiplatelet therapy and whether that was comparable to warfarin. Basically, NOACs > warfarin > dual antiplatelet > aspirin. However, there were more major bleeding events with dual antiplatelet therapy.