The fast-spreading delta variant has flooded hospitals across the South. It’s killed more people in Florida and Louisiana than the darkest days of the pandemic winter, and left so many COVID-19 patients gasping for breath that some places face shortages of medical oxygen.

This harsh reality, likely fueled by a failure to adequately vaccinate the most vulnerable, has undercut the best efforts of Florida Gov. Ron DeSantis and other Republican leaders to simply move past COVID.

For weeks, the virus preyed on America’s illusion of a defanged COVID. Most people returned to a semblance of their former lives, never suspecting that the country would revert to such levels of mortality.

Nationwide, mask use fell to a third of its previous peak. Traffic at restaurants and stores approached pre-pandemic norms. Big sporting events and music festivals returned. Now, public health experts say the U.S. needs to reconsider some of those changes as reported deaths from COVID-19 exceed 1,000 a day, and the nation looks warily ahead to another winter virus season.

“There was an underestimation of how penetrant delta could be,” said Vanderbilt University infectious disease professor William Schaffner. “There was a desire to really open things up again, and I think that worked contrary to good common sense.”

For weeks into early August, the delta surge in the U.K. lulled some Americans into a sense of complacency. Across the Atlantic, cases soared but killed relatively few people, and in theory, the mutated virus would act similarly in the U.S. But delta exposed a key difference: The U.S. has fallen far short of the U.K. in vaccinating the oldest members of the community, who remain most at risk of hospitalization and death.

That largely explains the U.S.’s failure: About 18% of Americans 65 and older still aren’t fully vaccinated, versus about 5% in the U.K. “That’s a huge difference,” said Jeffrey Morris, a professor of biostatistics at the University of Pennsylvania. “When you have four times as high a proportion that are unvaccinated, that’s going to cause a lot more death right there.”

The U.S. has already posted twice as many deaths per capita since early June as the U.K., even though its surge started later. In Florida, Mississippi and Louisiana, the tolls are 4-6 times as high – exacerbated by populations that tend to be older or have more pre-existing conditions.

Most states have prioritized vaccines for the oldest members of the community, but Republican leaders nationally and locally have muddied the waters in efforts to appeal to staunch anti-vaccination parts of their voter base.

Florida’s DeSantis and Texas Gov. Greg Abbott have emphasized residents’ freedom to choose whether they get vaccinated, signing bills to limit or block so-called vaccine passports. In Florida, the passport ban even pitted DeSantis against the state’s own cruise industry, a critical driver of jobs.

In the end, only Vermont, Connecticut and Maine are anywhere close to the 95% vaccination threshold for the people with the most dire need for inoculation.

DeSantis apparently thought the state’s efforts – which yielded a senior vaccination rate in line with the national average – were enough. On Aug. 3, he boasted to a crowd at a park-restoration news conference that mortality from COVID-19 was far lower than the previous year’s summer wave.

All the fuss about delta was fueled by media “hysteria,” he said. But DeSantis’s strategy failed to keep deaths from surging soon afterward. By Aug. 15, deaths in Florida had reached a seven-day average of 244 a day, exceeding the previous peak of 227 a year earlier.

Delta’s arrival coincided with the height of the U.S. mask wars. Across GOP strongholds in the South, governors fought to prohibit governments or schools from mandating facial coverings. DeSantis lost a battle last month when a group of parents prevailed in a lawsuit against the state. Florida is appealing.

Disputes over masks rage even after a massive new study of more than 340,000 people provided perhaps the best evidence yet of their lifesaving properties – rebuffing detractors who said the science just wasn’t sound enough.

The study conducted interventions in communities in Bangladesh, and managed to increase mask wearing by 29 percentage points compared with the project’s control group. The increased masking coincided with a 12% reduction in COVID-like symptoms, and the benefits were much greater still among the oldest and most vulnerable participants.

“This should totally end that specific argument” that the science is inadequate, said Jason Abaluck, a Yale University economist who coauthored the study.

But the U.S. has largely stopped using facial coverings, according to estimates published by the University of Washington’s Institute for Health Metrics and Evaluation. Universal masking would save 52,000 lives in the U.S. between now and Dec. 1, according to IHME.

For DeSantis in particular, the policies have helped make him a superstar in the GOP and on Fox News, and he’s frequently floated as a presidential candidate for 2024. He’s flooded the zone with beer koozies and T-shirts with a catchy slogan aimed at President Joe Biden’s top virus-fighting doctor, Anthony Fauci: “Don’t Fauci My Florida.”

But before DeSantis can run for president, he has a gubernatorial election next year. In an Aug. 17-23 Quinnipiac University poll, 61% of Florida voters said the recent surge in COVID-19 cases was preventable. Just over half disapprove of how DeSantis has handled the pandemic response, according to the poll. “The pandemic could eclipse all the other positives he has in the state,” said Tim Mallory, a Quinnipiac poll analyst.

In any case, it’s unlikely the U.S. has seen COVID’s final act.

As the country heads for an uncertain winter, one silver lining remains: It could have been much worse.

Morris, the biostatistics professor, noted that the vaccines are clearly preventing severe illness and death, and the COVID optimists may have been right about defanging COVID had it not been for the arrival of the delta variant.

But now the math has changed. “Whatever chinks there are in the armor,” he said, “delta is going to find them.”