In modern healthcare environments, a fundamental requirement for achieving continuity of care is the seamless access to distributed patient health records in an integrated and unified manner, directly at the point of care. However, Electronic Health Records (EHRs) contain a significant amount of sensitive information, and allowing data to be accessible at many different sources increases concerns related to patient privacy and data theft. Access control solutions must guarantee that only authorized users have access to such critical records for legitimate purposes, and access control policies from distributed EHR sources must be accurately reflected and enforced accordingly in the integrated EHRs. In this paper, we propose a unified access control scheme that supports patient-centric selective sharing of virtual composite EHRs using different levels of granularity, accommodating data aggregation and various privacy protection requirements. We also articulate and handle the policy anomalies that might occur in the composition of discrete access control policies from multiple data sources.