WASHINGTON — The summer that was supposed to mark America’s independence from COVID-19 is instead drawing to a close with the U.S. more firmly under the tyranny of the virus, with deaths per day back up to where they were in March.

The delta variant is filling hospitals, sickening alarming numbers of children and driving coronavirus deaths in some places to the highest levels of the entire pandemic. School systems that reopened their classrooms are abruptly switching back to remote learning because of outbreaks. Legal disputes, threats and violence have erupted over mask and vaccine requirements.

The U.S. death toll stands at more than 650,000, with one major forecast model projecting it will top 750,000 by Dec. 1.

“It felt like we had this forward, positive momentum,” lamented Katie Button, executive chef and CEO at two restaurants in Asheville, North Carolina. “The delta variant wiped that timeline completely away.”

It wasn’t supposed to be this way.

More than six months into the U.S. vaccination drive, President Joe Biden held a White House party on July Fourth to celebrate the country’s freedom from the virus, and other political leaders had high hopes for a close-to-normal summer.

Then the bottom fell out.

The summer wave was fueled by the extra-contagious delta variant combined with stark resistance to vaccinations that formed along political and geographic lines, said Dr. Sten Vermund, of the Yale School of Public Health.

“The virus was more efficient in spreading among the unvaccinated so that you blunted the expected benefit of vaccines,” Vermund said.

The crisis escalated rapidly from June to August. About 400,000 COVID-19 infections were recorded for all of June. It took all of three days last week to reach the same number.

The U.S. recorded 26,800 deaths and more than 4.2 million infections in August. The number of monthly positive cases was the fourth-highest total since the start of the pandemic.

The 2021 delta-driven onslaught is killing younger Americans at a much higher rate than previous waves of the pandemic in the Northeast last spring, the Sun Belt in the summer of 2020 and the deadly winter surge around the holidays.

During the peaks of those waves, Americans over 75 suffered the highest proportion of death. Now, the most vulnerable age group for death is 50 to 64.

Overall, the outbreak is still well below the all-time peaks reached over the winter, when deaths topped out at 3,400 a day and new cases at a quarter-million per day.

The U.S. is now averaging more than 150,000 new cases per day, levels not seen since January. Deaths are close to 1,500 per day, up more than a third since late August.

Even before the delta variant became dominant, experts say there were indications that larger gatherings and relaxed social distancing measures were fueling new cases.

“We had been cooped up for over a year and everyone wanted to get out,” said Dr. David Dowdy, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health. “In the face of that kind of strong change in behavior, even getting almost two-thirds of our adult population vaccinated wasn’t enough.”

The COVID-19 vaccines remain highly effective against hospitalization and death, but many tens of millions of eligible Americans remain unvaccinated. Nearly 40% of Americans 12 and older are not fully protected.

Yale’s Vermund sees reasons to be cautiously optimistic about the next few months. Cases in most states appear to be plateauing and are likely to decline in the fall, buying health authorities more time to vaccinate adults and teenagers before flu season.

“If we can continue making progress between now and Thanksgiving, we may be able to substantially blunt the coronavirus surge in flu season,” Vermund said.

On Wednesday, the head of the World Health Organization said rich countries with large supplies of coronavirus vaccines should refrain from offering booster shots through the end of the year and make the doses available for poorer countries.

Doubling down on an earlier appeal for a “moratorium” on boosters that has largely been ignored, WHO Director-General Tedros Adhanom Ghebreyesus also said he was “appalled” after hearing comments Tuesday from a top association of pharmaceutical manufacturers that vaccine supplies are high enough to allow for both booster shots for people in well-supplied countries and first jabs in poorer countries that face shortages. He said that’s already been the case.

“I will not stay silent when companies and countries that control the global supply of vaccines think the world’s poor should be satisfied with leftovers,” he told a news conference.