In the waiting rooms of the East Boston Neighborhood Health Center, bustling with a working-class clientele, doctors said much had changed since the state insurance law passed in 2006. People are less likely to put off care out of fear of unaffordable bills, and patients with diabetes can get medication regularly.

Dr. Stelios Maheras, medical director of the emergency department, said some patients used to ask for prices “like at the supermarket.” He recalled one patient who was having chest pains but refused an ambulance because he was afraid of the bill.

“I said, ‘You can’t drive yourself to the hospital — that’s a stress test all on its own!’” Dr. Maheras said. “The attitude has totally changed,” he said, adding that his patients now felt less financially vulnerable and more confident.

In Suffolk County, which includes Boston, the death rate for adults under 65 dropped by about 7 percent from 2005 to 2010, the study’s authors said.

There have been patchy efforts to improve coverage for the poor in states like Arizona, Maine and New York, but Massachusetts is the only state to fully overhaul its health system to cover almost everybody.

But studying the state in isolation was problematic: Was the mortality decline due to expanded insurance coverage, or other factors such as improved eating habits or lower smoking rates? The authors identified 513 counties in 46 other states that were most similar to Massachusetts before reform in demographics and levels of poverty and insurance, then compared their mortality rates with that of Massachusetts. They found that the rate declined 2.9 percent in Massachusetts, but remained flat in counties outside the state.