Not surprisingly, the CDC reports that the mandates dramatically increase the vaccination rate. U.S. hospitals that require flu shots reported a vaccination rate of 95.2 percent between 2011-12 season, compared with 68.2 percent at hospitals where they were not mandated, the CDC said.

Regardless, the practice rubs some labor groups the wrong way. Citing the "absence of a basic philosophic or personal exemption for healthcare workers," the Service Employees International Union released a statement opposing such workplace mandates. The SEIU, and its 1 million members, was joined in the statement by other unions, such as the AFL-CIO and National Nurses United. Critics also point to a recent study published in The Lancet, which found that the flu vaccine is 59 percent effective.

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