Deaths among children and adolescents became less common between 1990 and 2015, but not all countries benefited equally from the improvements, according to a new analysis.

Countries with low social and economic statuses shoulder a much larger child and adolescent mortality burden than do countries with better income, education and fertility levels, researchers found.

“The relative difference between the best and the worst is growing,” said Nicholas Kassebaum of the Institute for Health Metrics and Evaluation at the University of Washington in Seattle.

Kassebaum and dozens of other researchers in the Global Burden of Disease Child and Adolescent Health Collaboration analyzed data from 195 countries on children and adolescents through age 19.

The number of deaths in that age group fell from about 14.2 million in 1990 to about 7.2 million in 2015, the researchers write in JAMA Pediatrics.

Countries at all social and economic levels experienced a decline in child and adolescent death rates. But the proportion of those deaths that happened in countries at the lower end of the spectrum grew over time.

Specifically, countries with the lowest social and economic statuses accounted for almost 75 percent of all deaths among children and adolescents in 2015, compared with 61 percent in 1990.

The most common causes of death included complications after premature birth, lower respiratory tract infections and swelling of the brain.

The study can’t say why countries with lower social and economic statuses didn’t benefit as much over the years, but Kassebaum suggested some reasons.

For example, he said, countries made great strides over the years reducing infections and improving neonatal outcomes, but some countries have not been able to make those and other public health interventions universal.

Also, public-health advances in wealthier countries may not reach countries at the other end of the spectrum.

“Those advances haven’t been passed on to middle-income countries and certainly not to low-income countries,” Kassebaum said.

Christopher Sudfeld and Wafaie Fawzi of the Harvard T.H. Chan School of Public Health point out in an accompanying editorial that global deaths among children younger than 5 were cut in half between 1990 to 2015 but that the world is lagging in reducing the number of stillbirths and deaths among newborns.

“Additional financial and intellectual investments in adolescent health are also necessary to promote health behaviors and reduce risks that can have lifelong implications for adolescents, their families and their communities,” they write.