The CHADS2 score is a clinical prediction rule for estimating the risk of stroke in patients with non-rheumatic atrial fibrillation. The CHA2DS2-VASc score is a refinement of the CHADS2 score and extends the latter by including additional common stroke risk factors
Ref:
Validation of Clinical Classification Schemes for Predicting Stroke: results from the National Registry of Atrial Fibrillation". JAMA 2001;285(22):2864-2870

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The European Society of Cardiology (ESC) guidelines recommend that if the patient has a CHADS2 score of 2 and above, oral anticoagulation therapy (OAC) such as warfarin (target INR of 2-3) or one of the new OAC drugs, such as dabigatran) should be prescribed.
If the CHADS2 score is 0-1, other stroke risk modifiers could be considered:
(i) If there are 2 or more risk factors (essentially a CHA2DS2-VASc score score of 2 or more), OAC is recommended; and
(ii) If there is 1 risk factor (essentially a CHA2DS2-VASc score score=1), antithrombotic therapy with OAC or aspirin (OAC preferred) is recommended, and patient values and preferences should be considered.
A CHA2DS2-VASc score score=0 corresponds to a 'truly low risk,’ and thus the recommendation is to prescribe either aspirin or no antithrombotic therapy, but 'no antithrombotic therapy' is preferred.