Fewer than 3 percent of Massachusetts residents lacked health insurance last year, an all-time low in a state that served as a model for President Obama’s federal health care overhaul.

Figures released last week by the US Census Bureau show that at 2.8 percent, Massachusetts has fortified its longstanding position as the state with the lowest rate of uninsured residents. Nationally, 9.4 percent of people had no health insurance, down from 15.5 percent in 2010.

The uninsured rate had already been steadily declining in Massachusetts. For example, it stood at 4.4 percent in 2010, the year the federal Affordable Care Act went into effect.

Massachusetts enacted its own law to make health insurance accessible to nearly everyone years before the Affordable Care Act. So why would the state rate keep falling after introduction of the federal law?

Noah Berger, president of the Massachusetts Budget and Policy Center, a left-leaning nonprofit research organization, sees several likely factors.

One is the state’s booming economy, with the jobless rate at its lowest in 15 years. Most people get health insurance through their jobs, so if more people have jobs, more people have insurance.

Additionally, the publicity surrounding the federal law may have encouraged more Massachusetts residents to enroll in Medicaid or subsidized health insurance.

“Even though many of those people were previously eligible,’’ Berger said, “they may not have signed up.’’

A third factor is the expansion of MassHealth, the state’s Medicaid program, which has seen enrollment increase from 1.4 million in 2013 to 1.8 million in 2015. The biggest growth came in the category of poor adults who are not disabled, some of whom became eligible for Medicaid for the first time under the Affordable Care Act.

But while the Census numbers are interesting, “we have to interpret those with caution,’’ said Audrey Shelto, president of the Blue Cross Blue Shield of Massachusetts Foundation. Other studies also show Massachusetts with a low rate of uninsured residents — just not quite as low as in the federal review.

A survey commissioned by the Blue Cross foundation showed the percentage of uninsured at 4.3 percent in 2015, and holding relatively steady over the years. But that survey does not include children.

The state Center for Health Information and Analysis also has different estimates. The center reported that 3.6 percent of Massachusetts residents had no health insurance last year, virtually unchanged from the previous year. But the center’s survey used different methodology and had a wider margin of error than the Census.

Amy Whitcomb Slemmer, executive director of the advocacy group Health Care for All, has faith in the Census figures, saying they are “tracking a legitimate phenomenon.’’ She credits the state’s efforts to locate and enroll uninsured people.

But she cautioned that turmoil coming this fall could threaten that success. A large number of people are likely to exit a subsidized health plan that significantly raised its premiums for 2017. And MassHealth is expected to introduce new programs that will require consumers to make complex choices.

Amid the confusion, there is a risk some people may fail to re-enroll for 2017, Whitcomb Slemmer said. “It will take increased focus as things become more complicated,’’ she said.

Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her on Twitter @felicejfreyer