Covid-19 hospitalizations nationwide crossed above 100,000 this week for the second time in the pandemic, overwhelming caregiver capacity in several states.

Keeping ahead of demand is harder now than during earlier surges, according to doctors, nurses and hospital executives. Patients with other illnesses returned to hospitals this year, leaving fewer open beds as Covid-19 cases soared. The demand is most acute in ICUs, which care for the most-critical patients and need highly trained medical staff. Hospitals are short-staffed and unable to recruit enough nurses and respiratory therapists, who are exhausted as the pandemic wears on.

The climb in hospitalizations has been steep, fueled by the highly contagious Delta variant, hospital officials said. “That spike is more exaggerated,” said Hugh Tappan, who oversees 11 HCA Healthcare Inc. hospitals in Florida, Georgia and South Carolina. In recent weeks, they have paused surgeries that could be delayed, diverted ambulances and stopped taking patients from other hospitals as capacity has become strained.

Many hospitalizations can be prevented with vaccinations, hospital and public-health officials said. Immunizations are effective at preventing serious illness and death, research shows.

“We don’t have to have this many hospitalizations and deaths,” said Eric Toner, senior scholar at the Johns Hopkins Center for Health Security. “The way out, and we could do this tomorrow, is vaccination, masking and maintaining distance.”

Uneven vaccination rates across age groups have skewed younger the ages of patients hospitalized with Covid-19. Data show record rates of hospitalizations across age brackets for young adults and those in younger middle age. “The number of young, otherwise healthy people who are in hospitals is higher than we have ever seen before,” Dr. Toner said.

Older adults are more likely to be vaccinated, data from the Centers for Disease Control and Prevention show, and those ages 70 and older account for a smaller share of those hospitalized with the virus than earlier in the pandemic. Breakthrough cases that lead to hospitalization typically occur among people with weak immune systems and other medical conditions, according to hospital officials.

Vaccination rates rise with age, with about 60% of people fully vaccinated between ages 40 and 49 but only about half at ages 25 to 39, CDC data show. Chances of catching and spreading the virus are higher among younger adults, who are typically more social and active than the elderly, public-health experts said.

Native American, Black and Hispanic patients continue to be disproportionately hospitalized with Covid-19, and hospitalizations are growing fastest among Black patients across 99 counties in 14 states where the CDC collects such data, according to an analysis by researchers at the Harvard T.H. Chan School of Public Health.

The pandemic has had an outsize impact on Native American, Black and Hispanic communities. Provisional data from the CDC show that the virus lowered life expectancy more sharply for Black and Hispanic people than whites.

CDC data on vaccination rates by race and ethnicity nationally are incomplete; data are available for 63% of the fully vaccinated. Hesitancy and uncertainty over the shots is highest among people who identify as two or more races, according to the U.S. Census Bureau’s tracking of people who say they are definitely or probably not getting vaccinated or are unsure. Percentages are similar for Black and white respondents, but lower among Hispanic respondents, the tracking shows.

Lack of confidence in the shots isn’t the only reason some people are unvaccinated, said Dr. Thomas Tsai, an assistant professor of surgery and health policy at Harvard and one of the researchers who analyzed the data. Lack of access to shots is also an issue, he said, and one way that longstanding inequities in U.S. healthcare access have led to disproportionate hospitalizations among Native American, Black and Hispanic people, he said. “It’s just been writ large by the pandemic,” he said.

Georgia

In Georgia, Covid-19 hospitalizations are nearing a record, and intensive-care beds for the most critical patients are scarce. Statewide, about 95% of intensive-care beds are full, Health and Human Services data show. Some 55% are occupied by Covid-19 patients.

Before the Delta variant-driven surge, Coffee Regional Medical Center in Douglas, Ga., peaked at 58 Covid-19 patients in January. Most were ages 65 and older, and many had underlying health conditions, Chief Executive Officer Vicki Lewis said.

By Friday, the 98-bed hospital had 66 Covid-19-positive patients, including 44 in intensive care, overwhelming the 10 ICU beds, Ms. Lewis said. Overall, the hospital had 108 inpatients, leading it to convert meeting rooms and waiting rooms to care for patients.

All but three of the Covid-19 patients are unvaccinated, and those three were the least sick, Ms. Lewis said. The others are younger now, with a recent average age of 48, and they are generally otherwise healthy, she said.

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“Unfortunately our mortality rate has gone up, our incident rate has gone up, and the tragedy is the people who are passing are young,” Ms. Lewis said.

Roughly 43% of Georgia residents are fully vaccinated, according to the state Department of Public Health. Breakthrough infections make up less than 1%, or 25,040 cases, among those fully vaccinated, and 1,009 have been hospitalized through Aug. 24, the state data show.

She said people have been calling the hospital asking for vaccines for sick loved ones. Physicians have to tell them the shots are only a preventive measure, not a treatment.

Write to Melanie Evans at Melanie.Evans@wsj.com, Anthony DeBarros at Anthony.Debarros@wsj.com and Jon Kamp at jon.kamp@wsj.com