The pandemic inflicted higher rates of excess deaths on both Republicans and Democrats. But after COVID-19 vaccines arrived, Republican voters in Florida and Ohio died at a higher rate than their counterparts, according to a new study.

Researchers from Yale University who studied the pandemic's effects on those two states say that from the pandemic's start in March 2020 through December 2021, "excess mortality was significantly higher for Republican voters than Democratic voters after COVID-19 vaccines were available to all adults, but not before."

More specifically, the researchers say, their adjusted analysis found that "the excess death rate among Republican voters was 43% higher than the excess death rate among Democratic voters" after vaccine eligibility was opened.

The different rates "were concentrated in counties with lower vaccination rates, and primarily noted in voters residing in Ohio," according to the study that was published in the journal JAMA Internal Medicine on Monday.

It's the latest research to suggest the perils of mixing partisan politics with medical advice and health policy.

How was the study performed?

Researchers analyzed data related to 538,159 people who died between Jan. 1, 2018, and Dec. 31, 2021, at ages 25 and over, compiling their political party affiliations based on records from 2017.

The study collected weekly death counts, breaking down the deceased's party ties along with their county and age cohort. It used May 1, 2021, as a key dividing line because the date marks a month after all U.S. adults became eligible to receive shots of the COVID-19 vaccines.

The researchers estimated excess mortality based on how the overall rate of deaths during the pandemic compared to what would have been expected from historical, pre-pandemic trends.

Researchers saw a divide suddenly emerge

As they calculated excess death rate data for Florida and Ohio, the researchers found only small differences between Republican and Democratic voters in the first year of the pandemic, with both groups suffering similarly sharp rises in excess deaths that winter.

Things changed as the summer of 2021 approached. When coronavirus vaccine access widened, so did the excess death gap. In the researchers' adjusted analysis of the period after April 1, 2021, they calculated Democratic voters' excess death rate at 18.1, and Republicans' at 25.8 — a 7.7 percentage-point difference equating to a 43% gap.

After the gap was established in the summer of 2021, it widened further in the fall, according to the study's authors.

The study doesn't provide all the answers

The researchers note that their study has several limitations, including the chance that political party affiliation "is a proxy for other risk factors," such as income, health insurance status and chronic medical conditions, along with race and ethnicity.

The study focused only on registered Republicans and Democrats; independents were excluded. And because the researchers drilled into data in Florida and Ohio, they warn that their findings might not translate to other states.

The researchers' data also did not specify a cause of death, and it accounts for some 83.5% of U.S. deaths, rather than the entire number. And because data about the vaccination status of each of the 538,159 people who died in the two states wasn't available, researchers could only go as granular as the county level in assessing excess deaths and vaccination rates.

The study was funded by the Tobin Center for Economic Policy at Yale University and the Yale School of Public Health COVID-19 Rapid Response Research Fund.

New findings join other reviews of politics and the pandemic

In late 2021, an NPR analysis found that after May of that year — a timeframe that overlaps the vaccine availability cited in the new study — people in counties that voted strongly for Donald Trump in the 2020 presidential election were "nearly three times as likely to die from COVID-19" as people in pro-Biden counties.

"An unvaccinated person is three times as likely to lean Republican as they are to lean Democrat," as Liz Hamel, vice president of public opinion and survey research at the nonpartisan Kaiser Family Foundation, told NPR.

Even before vaccines were widely accessible, researchers were working to quantify the effects of vastly divergent COVID-19 policies across U.S. states.

A widely cited study from early 2021 found that in the early months of the pandemic's official start date in March 2020, states with Republican governors saw lower COVID-19 case numbers and death rates than Democratic-led states. But the trend reversed around the middle of 2020, as Republican governors were less likely to institute controls such as stay-at-home orders and face mask requirements.

"Future policy decisions should be guided by public health considerations rather than by political ideology," said the authors of that study, which was selected as the article of the year by the American Journal of Preventive Medicine.