Covid-19 case rates are once again rising across the United States, which has experts concerned about a potential winter case surge, particularly in areas with low vaccination rates.

Your top resources for Covid-19 readiness

'A new winter surge'

According to CDC data, daily Covid-19 cases in the United States have increased nearly 27% over the last three weeks. As of Nov. 14, the seven-day daily case average was 80,823, up from 63,852 cases on Oct. 24.

How much worse will the 'delta surge' get? Watch these 7 factors.

Currently, 29 U.S. states and territories are seeing increases in Covid-19 cases, NBC News reports. In particular, the Midwest and Northeast are now seeing growing outbreaks after largely avoiding a surge during the summer, NPR reports.

As cases continue to rise across the country, experts warn that this could be the start of an extended winter Covid-19 surge. "I hate to say it, but I suspect we're at the start of a new winter surge," said George Rutherford, an epidemiologist at the University of California, San Francisco.

He added, "There are still large swaths of the country under-immunized and even among states that are relatively well-vaccinated, like Colorado, New Mexico, Minnesota, and Vermont, we're seeing sustained transmission."

According to NBC News, Vermont, which has a 72% vaccination rate, has seen a 60% jump in new Covid-19 cases over the past few weeks. Jan Carney, associate dean for public health at the University of Vermont’s Larner College of Medicine, said there have been some breakthrough infections, but the majority of new Covid-19 cases have been from the delta variant spreading among those who are unvaccinated.

New Hampshire has also seen a significant rise in Covid-19 cases over the last few weeks, and a large portion of these cases has been among younger people who are less likely to be vaccinated, NBC News reports.

"If you look at the current surge, almost one-third of the new cases are in younger age groups," said Aalok Khole, an infectious disease physician at Cheshire Medical Center in New Hampshire. "People between ages 12 and 35 dominate new cases right now. And a lot of this population has not been vaccinated by a single dose."

What could a winter surge in Covid-19 cases look like?

According to NPR, the severity of a potential winter surge in the United States depends on "complex dynamics around immunity." For example, the summer surge experienced by southern states may have helped them build up immunity that could prevent a surge in the winter. In contrast, other areas that did not experience a summer surge, such as many northern states, are now seeing increases, although higher vaccination rates may offer some protection.

"I think the Midwest and the Great Plains—which have lower vaccination rates but have not seen a big delta surge—they may very well end up seeing quite a few infections in the weeks and months ahead," Jha said.

However, despite concerns about rising Covid-19 case rates, experts who model the pandemic's course said that a winter surge is not likely to result in the same level of hospitalization and death as last year, NPR reports.

"The vast majority of the population has some form of immunity," said Nicholas Reich, a biostatistician who runs a Covid-19 forecast model at the University of Massachusetts Amherst. "That feels really different about this moment—there are fewer people to infect."

Hospitals may struggle even with a small winter surge

Although the winter surge may not be as severe as expected, even a small surge in Covid-19 cases may overwhelm hospitals, NPR reports. Many hospitals are already struggling with staff shortages and packed ICUs, and some have had to activate crisis standards of care.

"Not only are your health care resources getting stretched yet again because of rising numbers of hospitalizations, but we're also battling other issues, and nationwide everyone is going through a shortage of health care workers, burnout, and resiliency issues," Khole said. "How we're going to fare through the winter is probably a concern in every health care worker's mind right now."

Separately, Kencee Graves, a physician at the University of Utah Hospital, said the hospital shut down its surge ICU due to a lack of staff, leaving patients to wait between three and five hours for an ICU bed. "Our resources and our stamina are far less now than a year ago," she said.

However, some actions may help limit the effects of a winter surge, such as "more vaccines and kids getting vaccinated," Jha said. "So we're in a stalemate. I don't expect us to have a horrible surge, but I can certainly imagine parts of the country that see modest-sized surges as people get together [for the holidays] and as the weather stays cold." (Stone, "Shots," NPR, 11/16; Murphy et al., NBC News, 11/15; Breslin, The Hill, 11/16)