UnitedHealth Group Inc, the largest U.S. health insurer, on Wednesday backed Gilead Sciences Inc's Harvoni as the preferred hepatitis C treatment on its 2015 commercial drug coverage list, another victory for Gilead over competitor AbbVie Inc. Gilead and AbbVie have been battling for hepatitis C market share since the approval in December for an AbbVie drug that challenges Gilead's blockbusters. Insurers and pharmacy benefit managers have been striking deals for preferred or exclusive status in exchange for discounts for 2015.