Delta accounts for nearly all virus cases in southwestern Missouri, where a hospital recently ran...

The crush of new Covid patients this week at Mercy Hospital in Springfield, Mo., was unlike anything the staff had seen during the past 16 months. Officials resorted to borrowing ventilators from other hospitals and pleaded on social media for help from respiratory therapists.

The run on ventilators subsided, but this week Mercy Springfield cared for more Covid patients than it has seen in any week since the pandemic started. The city is in Greene County, where only about 35 percent of residents have been fully vaccinated and the highly contagious Delta variant is spreading.

“Six weeks ago, we had 10 patients and now it’s 128 on Friday, which is greater than the third wave back in December,” said Dr. John Mohart, Mercy’s senior vice president of clinical services.

Missouri has been averaging about 1,000 new cases per day, a 44 percent increase over the past two weeks, though a fraction of the state’s November peak when its average topped 5,000, according to a New York Times database. Hospitalizations are up 25 percent from two weeks ago.

The Delta variant of the coronavirus has driven this outbreak in Missouri’s Southwest, around Springfield and Joplin, according to the Centers for Disease Control and Prevention. This week, an epidemiologist with the C.D.C. arrived in Springfield, as part of a “surge response team” requested by the state.

Delta became the dominant variant in the United States this week, accounting for about 52 percent of cases. The C.D.C. estimates the variant was behind nearly three-fourths of the new cases in Missouri.

Studies suggest that vaccines remain effective against the Delta variant. But public health experts say Delta poses a serious threat to unvaccinated populations, and studies suggest that a single shot of a two-dose vaccine regimen provides only weak protection against the variant.

At the same time, the gap in vaccination rates between counties that voted for Donald J. Trump and those that voted for Joseph R. Biden Jr. is widening, according to a new survey released by the Kaiser Family Foundation.

Missouri has a vaccination rate of 40 percent, well below the national rate of 48 percent. Many counties have vaccination rates in the teens and 20s.

The number of Missourians seeking the vaccine has dwindled. Nearly 50,000 were getting vaccinated each day in mid-April; on Thursday, it was 7,000.

The drop off in vaccinations is true across the country. As of Friday, providers were administering about 590,000 doses per day on average, about a 82 percent decrease from the peak of 3.38 million reported on April 13.

Hospitalizations are rare among the fully vaccinated. Dr. Mohart said that 95 percent of the 128 Covid patients at Mercy Springfield on Friday had not been immunized and tests show that most cases were of the Delta variant.

It was a similar picture at Cox Medical Center South in Springfield, which admitted 19 new patients for Covid-19 treatment on Thursday. From June 1 through July 8, there were 29 coronavirus deaths there.

“The staff is exhausted and overwhelmed,” said Ashley Casad, vice president of clinical services for CoxHealth, which has six hospitals in southwest Missouri and northwest Arkansas. “This is going to go on for a month. The light at the end of the tunnel seems incredibly far away.”

She said the rise in cases seemed to be a caused by three factors: the area’s low vaccination rate, the arrival of the Delta variant and Springfield’s recent decision to lift its mask mandate. Ninety percent of Covid patients at the Cox Medical Center South in Springfield have the Delta variant, and they are trending younger, she said.

Ms. Casad said the hospital was sending patients to other medical centers because it lacked enough staff to care for them, though the supply of respirators was adequate.

A state health official said on Friday that 500 ventilators were stockpiled in the capital, Jefferson City, and were available to be sent out to hospitals, but that so far there had been no requests for them.

The rise of new cases in southwest Missouri prompted Springfield public schools to mandate masks for summer session, and Mercy health system said that it would require its staff to be immunized by Sept. 30.