The death toll among incarcerated people in the U.S. increased at a rate more than three times higher than among the general population during the worst phase of the COVID-19 pandemic, researchers say.

Why it matters: Although it's long been clear that prisons struggled to contain COVID outbreaks, there's still no official pandemic prison death toll — leaving the work up to a "patchwork of research groups and reporters," per a new report from the Marshall Project, a nonprofit criminal justice news outlet that analyzed the findings.

Driving the news: Mortality in U.S. prisons increased 77% in 2020 compared to 2019 — 3.4 times the rise among the free population, per a study published in Science Advances.

• The researchers, out of the University of California, Irvine, and Brigham and Women's Hospital, combined disparate state and federal prisons data to create what they consider "the most comprehensive understanding to date of in-custody mortality during 2020."

Between the lines: The pandemic hit older prisoners especially hard, per the Marshall Project.

• There were 150 more deaths per 10,000 incarcerated people in 2020 compared to 2019 among those 65 or older, but about one more death for those 49 and under.

The bottom line: Understanding COVID's true impact in prisons is key to learning "from what happened, so we don't do this again in the future when we have another pandemic, another crisis," study lead author Naomi Sugie told the Marshall Project.