The health care reform law has so far had little effect on the percentage of employees enrolled in employer-sponsored health care plans, according to a survey.

The Mercer L.L.C. survey of nearly 600 employers found that on average 83% of employees eligible for coverage enrolled in plans this year, down from 84% in 2014.

In addition, the percentage of employees — both eligible and ineligible for coverage — who enrolled in plans was unchanged at 74% in both 2014 and 2015, according to the survey, which was released Tuesday.

That lack of enrollment growth is somewhat of a surprise given the stiff penalties that employers and employees face under the health care reform law if they do not, respectively, offer coverage or enroll in a plan.

Employers with at least 100 employees that do not offer coverage in 2015 to at least 70% of full-time employees — those working an average of 30 hours or more a week — are liable for a $2,000 per employee penalty. In 2016, the penalty applies to employers with at least 50 employees and coverage has to be offered to at least 95% of employees.

Employees not enrolled in a health care plan are liable in 2015 for a penalty of $325 or 2% of income, whichever is greater.

“Employers that had to offer coverage to more employees were braced for a bump in enrollment this year,” Tracy Watts, a senior partner and national leader for health reform in Mercer’s Washington office said in a statement.

One reason, Mercer believes, that employers did not see enrollment growth is that some employees opted out of coverage from their employers and instead enrolled in Medicaid.

The health care reform law gave states funds to expand Medicaid to more lower-income individuals, and about two dozen states took advantage of that offer.

According to the Mercer survey, 14% of respondents with 5,000 or more employees said they believe some employees who previously opted for coverage now waived it and instead enrolled in Medicaid.

In addition, some employers took steps to hold down enrollment growth by reducing employees’ hours below the 30-hour threshold that triggers the reform law’s penalty for not offering coverage.

In fact, 16% of respondents require that newly hired part-time employees work less than 30 hours a week, while 7% said they reduced hours of part-time employees who consistently worked at least 30 hours a week.