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The decades-long progress in reducing premature deaths from heart disease in the United States has slowed over the last decade, according to a new county-by-county analysis that found an increase in disparities affecting African American adults and younger populations.

U.S. and Chinese researchers used multiple databases that included information on nearly 1.6 million premature cardiac deaths between 1999 and 2017, among people ages 35 to 74. Though rates declined during the full study period, the pace of decline has slackened significantly since 2010, according to a study published Wednesday in the .

"The United States experienced remarkable decline in cardiovascular disease mortality that was coined as one of the major public health accomplishments of the 20th century," lead investigator Dr. Zhi-Jie Zheng said in a news release. He is chair of the department of global health at Peking University in Beijing.

"Increasing numbers of out-of-hospital deaths and fatal heart attacks in younger age groups, coupled with a steady widening of disparity of socioeconomic and health environment factors affecting health care at the county level, appear to be the key drivers of the slowdown," said Zheng, a former lead epidemiologist at the Centers for Disease Control and Prevention in Atlanta and a supervisory medical epidemiologist at the National Heart, Lung, and Blood Institute in Bethesda, Maryland.

Among the study's findings:

– 61% of the nearly 1.6 million premature cardiac deaths from 1999 to 2017 were not in a hospital.

– Out-of-hospital deaths rose from 58.3% in 1999 to 61.5% in 2017.

– Twice as many premature cardiac deaths occurred in men compared to women.

– Premature death rates from heart disease were 3.4 times higher among African Americans than Asians or Pacific Islanders.

The findings are alarming, Zheng said, and warrant more efforts to integrate lifestyle and behavior changes that boost heart health.

"Our findings suggest a need for health care policy changes and programs that can identify high-risk, young populations prone to premature cardiac death and support improved cardiac health," he said.

The disparities were largely linked to demographics and socioeconomic status, according to the study.

Differences in population size, rural living, sex, age, race/ethnicity and foreign-born status accounted for just over a third of out-of-hospital and in-hospital premature cardiac deaths. Disparities in factors such as median household income, unemployment, school enrollment and number of violent crimes per 100,000 population accounted for about 20% of deaths.

The study is among the first to analyze county-level factors related to disparities in premature cardiac death rates.

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