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A Kaiser Permanente Health Plan data breach in mid-April affected 13.4 million members in what the U.S. Department of Health and Human Services called the largest confirmed health-related breach so far this year. Kaiser Foundation Health Plan, which operates as Kaiser Permanente, is one of the leading U.S. health care providers.

Companies covered by HIPAA are required to notify HHS of data breaches involving protected health information, such as medical data and patient records. The company submitted the required documentation to the agency on April 12, and the notice was posted publicly last week. Kaiser spokesperson Diana Yee said the organization would begin notifying affected current and former members and patients who accessed its websites and mobile apps. The notifications will start in May in all markets in which Kaiser Permanente operates. California-based Kaiser also notified its state attorney general of the breach.