Governor Ricardo Rossello of Puerto Rico raised the island's official death toll from Hurricane Maria from 64 to 2,975 following a new, government-commissioned study carried out by George Washington University.

The big picture: The new study, released Tuesday, found that Hurricane Maria and its aftermath resulted in an estimated 2,975 deaths on the island from September 2017 through February 2018. However, it also noted that excess, storm-related deaths may have continued beyond that period.

The new mortality count means Hurricane Maria killed more people than Hurricane Katrina in 2005, which caused 1,833 fatalities.

According to the new study, the original figure stemmed from the government's classification of deaths directly attributable to the storm — like those caused by "structural collapse, flying debris, floods and drownings" — rather than those that occurred in the hurricane's aftermath.

The Details: Deaths in the wake of the Category 4 storm were caused in part due to the complete collapse of the power grid and lack of health care services in rural areas of the island. Previous studies by other research groups also found a far higher death toll than the government initially estimated.

How they did it: A key component of the study was an excess mortality study, which involved analyzing past mortality patterns from 2010 to 2017 in order to predict the expected number of deaths had Hurricane Maria not occurred, and comparing this to actual deaths observed.

The researchers also needed to project forward mortality with and without the hurricane, which required taking account for the massive migration off the island in the storm's wake. For observed deaths, the study relied on records for all deaths between September 2017 and February 2018 from the Puerto Rico Department of Public Health.

What they found: The study estimated that total excess deaths after the hurricane, when accounting for off-island migration, amount to 2,975 for the September 2017 through February 2018 period.

“The results of our epidemiological study suggest that, tragically, Hurricane Maria led to a large number of excess deaths throughout the island. Certain groups – those in lower income areas and the elderly – faced the highest risk,” said Carlos Santos-Burgoa, the principal investigator of the project and a professor of global health at GW Milken Institute School of Public Health.

Interestingly, the data showed that every age group and social group saw excess deaths, but that the risk of death was 45% higher for people living in poorer areas, and older males in particular also saw a continuously elevated risk of death during the study period.

As for the government's low death count, the study found that medical professionals had not been trained in how to fill out death certificates in a post-disaster situation, and hesitated to attribute fatalities to the storm either directly or indirectly.

"Our study shows that physician lack of awareness of appropriate death certification practices after a natural disaster and the Government of Puerto Rico’s lack of communication about death certificate reporting prior to the 2017 hurricane season limited the count of deaths that were reported as related to Hurricane Maria," the report says.

An April study published in Health Affairs showed that the lack of proper scientific data collection in Puerto Rico during and after Category 4 Hurricane Maria hit the island last September resulted in the death toll being severely underreported, which limited necessary financial and other aid resources.

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