On March 9th, the Office of the National Coordinator (ONC) and the Centers for Medicare and Medicaid Services (CMS) released coordinated rules that establish significant new  requirements for health IT developers, providers and payers to allow wide-ranging access to data through public application programming interfaces (APIs) and prevent information blocking. The final rules are designed to encourage widespread use of third-party, consumer facing tools by patients to access and use their health data. The rules also define limited exceptions to the requirements to share data with third-parties acting on behalf of patients.
Summaries of the rules are attached (ONC and CMS). The full rules are available here and here.