"We are concerned that those costs could creep higher," Deutsche Bank analyst Paul Trussell told CNBC.
Wal-Mart offers health-insurance coverage to both full-time and part-time employees, with some plans costing as little as $18 a paycheck, according to a company spokesman.
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One factor that is likely driving more employees to seek benefits is the Affordable Care Act's mandate that most individuals have some form of insurance coverage or be subject to a tax penalty. Workers likely weighed their options for getting coverage either through their employer or through an Obamacare health exchange.
In addition, in some states, some low-wage workers may not have qualified for a subsidy for their health coverage because their state chose to not expand access to Medicaid. Under the health-reform law, individuals who earn less than 200 percent of the poverty level couldn't qualify for a subsidy. Instead, these people were eligible to receive coverage through Medicaid—but that was only if their state opted for the Medicaid expansion.
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Wal-Mart said it anticipated that more workers would seek health coverage, but the actual numbers topped their projections. Trussell said that with the year now halfway complete, it's "fair to say they probably have a better gauge now just on how many associates of theirs will participate in the plan."
Although he said the retailer will more than likely face this headwind all year, he views it as mostly one-time in nature. Trussell has a "hold" rating and $75 price target on the company.