Plan Sponsor for HIPAA privacy purposes

[Guest sbock]

If an employer, in this case a city, has a contract with a union to pay all or part of the premiums for an HMO plan that is "an alternative to the city's group health plan," and the union negotiates with the HMO and has all other contact with the HMO, is the City or the union the "plan sponsor" for HIPAA purposes?

Only the union rep talks with this HMO, and if any HMO participant has a problem, they talk to the union rep for help (the union rep is a city employee, but is not part of the city's human resources/benefits department).