The rate of gun deaths in the United States has experienced an uptick that started four years ago, a new study says — the first significant increase in these fatalities in 15 years.

Since 1999, the number of gun deaths held steady year after year — at 10.4 firearm fatalities per 100,000 people. But in 2015, the rate began creeping up nationwide to 11.8 deaths per 100,00 people–marking a 13.8-percent increase.

According to an analysis of death certificate data from the Centers for Disease Control and Prevention, published Monday in the journal Health Affairs, more than 610,000 people died in the U.S. as the result of a firearm between 1999 and 2017–the most recent year available. There were more than 114,600 firearm deaths from 2015 to 2017 — accounting for nearly a quarter of all gun-related deaths since 1999. Almost 59 percent of those deaths were determined to be suicides, while another nearly 39 percent were tied to homicides.

“Clearly, we’re an international outlier.”

The numbers also reveal wide variation among the states. Forty states saw a rise in gun death rates, with the most dramatic increases in North Dakota, Missouri, Ohio and New Hampshire. States that showed a significant drop in gun death rates were Arizona, California, Nevada and New York, as well as the District of Columbia.

The study did not offer any reasons to explain why gun deaths have increased in specific states, or gone down in others. While the overall national increase could be the result of “random variation,” the study’s authors wrote that, “the consistency we found across states and subpopulations makes that possibility less likely.”

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When it comes to gun violence data, it is no accident that unanswered questions loom over the U.S., said Garen Wintemute, chair of the Violence Prevention Research Program at the University of California, Davis. When motor vehicle deaths were on the rise nationwide in the 1950s and 1960s, money fueled research to bring down the number of people who died. That response is very different from how the country has confronted gun violence in recent decades.

“Firearm violence is unique,” Wintemute told the PBS NewsHour in September. “The federal government has chosen consciously, deliberately to not study this thing.”

Part of why the U.S. does not capture gun injury data is tied to the 1996 Dickey Amendment, where Congress said the nation could not fund research that could be used to advocate against firearms. The result had a chilling effect on studies launched by the federal government that remains decades later.

While the House passed language to clarify the Dickey Amendment in 2018, the Senate stalled on passing legislation that could have funded more studies to better understand the toll of gun violence.

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A full picture of the nation’s health effects from firearms is hard to capture. The U.S. has more guns per civilian than any other country in the world, and it’s impossible to imagine the sheer number of firearms in the U.S. has no relationship with volume of gun deaths nationwide, said Deborah Azrael from the Harvard Injury Control Research Center.

“Clearly, we’re an international outlier,” Azrael said.

This latest study does not include data on non-fatal gun injuries because the federal government does not collect it nationwide.

“There’s a lot of information missing when you only look at mortality.”

Jason Goldstick, the study’s lead author and a research assistant professor in the Department of Emergency Medicine at the University of Michigan, said many of his colleagues are emergency room physicians who treat survivors of gunshot wounds. Those injuries are worth studying, too, because they present their own very real costs, such as medical care, disabilities and lost wages and productivity, Goldstick said.

Harnessing existing data systems to capture a broader picture of the health burden of guns in the U.S. “really should be a priority,” Goldstick said.

“There’s a lot of information missing when you only look at mortality,” he said.

But that may change. Outgoing Sen. Johnny Isakson, R-Ga., proposed in September the “Expanding Research on Mass Violence Prevention Act” to give the CDC $75 million each year to improve the collection and reporting of the National Death Reporting System, along with a system that could track perpetrators of mass violence and possibly intervene before such violence occurs.

The funds would “improve upon existing data systems that can track deaths and injuries associated with mass violence by improving data timeliness, the ability to collect detailed circumstantial information on these incidents,” according to CDC spokesperson Courtney Lenard. The changes, she said, would also enable the agency to assess when mass violence is “more or less likely to occur.”

But “no simple global policy is going to prevent all of these deaths,” Goldstick said. If policymakers want to reduce gun violence, he added, they have to get creative and understand one size does not fit all.