UnitedHealth Group's stock price fell 0.46%. Data from the U.S. Department of Health and Human Services' website on Thursday (August 14) showed that a cyberattack on the company's technology subsidiary last year affected 192.7 million people. The company estimated in January that the Change Healthcare system breach compromised the personal information of 190 million people.

This, the largest healthcare data breach in the United States to date, was revealed in February of last year. The hacker group "Black Cat" compromised the subsidiary's systems with ransomware, disrupting claims processing nationwide and impacting both patients and healthcare providers.

The number of people affected is listed on a data breach list maintained by the U.S. Department of Health and Human Services' Office for Civil Rights. UnitedHealth Group did not immediately respond to a request for comment.

Sensitive information believed to have been compromised includes health insurance member IDs, patient diagnostic records, treatment information, Social Security numbers, and billing codes used by healthcare providers.