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The Affordable Care Act has prompted few large companies to change or begin offering health insurance plans to their employees — because most already comply with the president’s signature healthcare law, according to Mercer, a national human resources firm.

Mercer said it found that only 16% of large firms (at least 50 employees) made changes in 2015 to comply with the ACA’s mandate for employees who work 30 hours or more, according to published reports. Results of the 572-company survey found that nearly 200 of them reported covering a higher percentage of employees. Companies with 50 to 99 employees will begin falling under the mandate in 2016.

The employer-sponsored mandate took effect for companies of 100 employees or more on Jan. 1 of this year, forcing penalties unless employers covered at least 70% of their staff.

A Mercer executive said survey results also revealed many employees without employer-sponsored health coverage were insured either under parents’ or spouses’ health plans.