(Reuters) - 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. UnitedHealth's drug coverage list, which is effective Feb. 1, applies to all of the company's commercial, fully insured customers who use its Optum RX pharmacy benefit manager and self-insured customers such as large employers who follow the drug list. The update to the formulary, available on the company's website, also applies to its Medicaid, Medicare and military contracts. "We are constantly working to deliver value and manage costs for customers and consumers throughout our business," UnitedHealth spokesman Tyler Mason said in a statement. Gilead declined to comment. Gilead set off a firestorm last year when it launched a breakthrough treatment for hepatitis C called Sovaldi that carries an $84,000-per-treatment price tag. Insurers, including UnitedHealth, were taken by surprise by its cost and called it out as an unexpected hit to profits last year. Express Scripts, a pharmacy benefit manager, pushed back against the price of Sovaldi and Gilead's follow-up drug Harvoni, which has a list price of $93,400. It backed AbbVie's Viekira Pak, saying it had gotten a significant discount. Analysts and investors say the treatments are being discounted by at least 30 percent. The companies have declined to comment. UnitedHealth is the last of the top three U.S. insurers to strike a deal with either Gilead or AbbVie. Reuters reported two weeks ago that Aetna Inc had backed Gilead, as have Anthem Inc and Humana Inc. Pharmacy benefit manager CVS Health also went with Gilead while smaller competitor Prime Therapeutics kept both on its list. The state of Missouri earlier this week said it had selected AbbVie's Viekira Pak for Medicaid patients who meet certain criteria, and said the agreement would reduce treatment costs by 30 to 40 percent. UnitedHealth has 45 million medical customers. Of those, 18 million are employed by companies that are self-insured and use UnitedHealth to administer their plan benefits; not all those employers follow UnitedHealth's commercial drug guideline or use UnitedHealth for their drug benefits. Gilead shares, which closed down 2.5 percent at $102.45, were trading slightly higher in after-hours activity at $103.11.