Which of the following is considered a breach under HIPAA?

A breach under HIPAA is defined as an impermissible use or disclosure of protected health information (PHI) that compromises the security or privacy of that information. The accidental disclosure of PHI fits this definition because it involves the unintended release of patient information, which can lead to the unauthorized access or usage of sensitive health data. Such an incident can result in harm to the patient and violates the HIPAA privacy and security rules, necessitating reporting and potential remediation efforts.

In contrast, the other options listed do not fall under the definition of a breach according to HIPAA guidelines. Late submission of health claims is an administrative issue and does not involve the unauthorized use of PHI. Likewise, the inaccessibility of medical records is more about system functionality or availability rather than a direct violation of PHI protection. Lastly, failure to update patient contact information deals with data management and accuracy rather than the exposure or mishandling of protected health information. Thus, only the accidental disclosure of PHI constitutes a breach as defined by HIPAA.