Originally Published: 13 JAN 22 03:47 ET

Updated: 13 JAN 22 14:08 ET

By Travis Caldwell and Jason Hanna, CNN

(CNN) -- As a record number of Americans are infected with Covid-19, largely due to the rapidly spreading Omicron variant, some states' health care systems are beset with nearly full intensive care units.

Nineteen states have less than 15% remaining capacity in their ICUs. Four of them have less than 10%: Kentucky, Alabama, Indiana and New Hampshire, according to data Wednesday from the US Department of Health and Human Services.

The other states are: Arizona, Delaware, Georgia, Massachusetts, Mississippi, Missouri, Nevada, New Mexico, North Carolina, Ohio, Oklahoma, Pennsylvania, Rhode Island, Texas and Vermont, according to HHS.

Nationally, the number of people hospitalized in the US with Covid-19 has reached a record high -- 151,261 as of Wednesday. And as infection spreads, states and health care systems nationwide are handling shortages of available medical workers, who face a greater chance of Covid-19 exposure and must isolate after testing positive.

National Guard personnel and other federal emergency teams have been sent to hospitals and long-term care facilities in places including New Hampshire to alleviate burdens with medical and non-medical tasks. And a new wave of federally deployed medical teams will head soon to six states -- Michigan, New Jersey, New Mexico, New York, Ohio and Rhode Island -- to help hospitals combat Covid-19, President Joe Biden announced Thursday.

"This is part of the winter surge, part of the long haul, which is why we put so many of the mitigation strategies and measures in place early on to help provide some flexibility to hospitals and health care systems," New Hampshire Gov. Chris Sununu said Wednesday.

Early research indicates the Omicron variant may produce less of a chance of needing hospitalization than prior Covid-19 variants. But Omicron's increased transmissibility means more people who are at higher risk for severe disease, such as those who are unvaccinated or immunocompromised, will be infected.

"Omicron continues to burn through the commonwealth, growing at levels we have never seen before. Omicron is significantly more contagious than even the Delta variant," Kentucky Gov. Andy Beshear said Monday. "If it spreads at the rate we are seeing, it is certainly going to fill up our hospitals."

While conditions are not as dire as at the start of the pandemic nearly two years ago due to the availability of vaccines and other treatment options, the staffing shortages in hospitals are a real concern during this latest surge, said Dr. Craig Spencer, director of global health in emergency medicine at New York-Presbyterian/Columbia University Medical Center.

"The problem is that right now we have hospitals where there's not enough nurses to take care of the patients who are coming in, the Covid patients and the non-Covid patients," Spencer told CNN's Laura Coates on Wednesday.

"That's exactly why we need to do everything we can to try to limit the number of people that are infected, not just those that are older or unvaccinated or not boosted, but everyone. Because each infection represents a potential to infect more people. We need to do what we can to slow that spread right now and ease the pressure on our hospitals," Spencer said.

For those who come into emergency rooms for non-Covid reasons yet test positive, hospitals are still having to invoke quarantine protocols for those patients which puts a strain on operations, he said. And that can have an effect on all patients.

"Right now, we're still seeing sick people that need oxygen, the overwhelming majority of which are unvaccinated. But a lot of the patients that we're seeing right now have underlying chronic conditions that are being exacerbated," Spencer said.

Those patients, he said, can include "someone who gets Covid is dehydrated and needs to stay in the hospital, or someone who gets Covid and is too weak and they can't go home because they're a fall risk. Those aren't as bad in one sense as those kind of classic Covid patients we were seeing before. But every single patient that needs to stay in the hospital takes up a bed. And beds and staffing are what's in short supply right now."

Share of hospitalizations from breakthrough infections is growing, but risks for unvaccinated are higher

Fully vaccinated people are accounting for a growing share of people hospitalized with Covid-19 -- but hospitalizations among people who received a booster shot still are rare, and the gap in risk by vaccination status has been wide.

Between April and July 2021, before the emergence of the Omicron variant, more than 90% of Covid-19 hospitalizations were among people who were either unvaccinated or partially vaccinated, according to a study published by the US Centers for Disease Control and Prevention.

But a sampling of data collected by CNN suggests that figure has dropped to somewhere between 60% and 75% in recent days and months:

• In Pennsylvania, about 75% of Covid-19 hospitalizations between September and early December 2021 were among people who were not fully vaccinated, according to data from the state health department.

• In New York, about 61% of Covid-19 hospitalizations during the week ending January 2, 2022, were among people who were not fully vaccinated, according to data from the state health department.

• Beaumont Health, the largest health care system in Michigan, reported last week that 62% of Covid-19 patients in its eight hospitals were unvaccinated.

While fully vaccinated people are accounting for a larger share of Covid-19 hospitalizations, multiple accounts suggest that those who are fully vaccinated and boosted account for a small share.

In the University of Maryland Medical System, less than 5% of hospitalized patients were fully vaccinated and boosted, an official there said last week. Beaumont Health reported last week that only 8% of Covid-19 patients were fully vaccinated and boosted.

The CDC did not respond to CNN's multiple requests for data on the share of Covid-19 hospitalizations by vaccination status.

The agency publishes data on its website regarding the relative risk by vaccination status. Cumulatively, the risk of hospitalization has been eight times higher for unvaccinated people than for fully vaccinated people. But in the last week of November, CDC data showed that hospitalization rates were about 17 times higher for unvaccinated people than for fully vaccinated people.

CDC to update mask guidance

Health experts are reiterating the need to wear quality masks as never-before-seen figures of positive Covid-19 cases strike the country.

The US averaged more than 771,580 new Covid-19 cases daily over the past week, according to Johns Hopkins University data, more than three times that of last winter's peak average (251,987 on January 11, 2021) and more than 4.5 times the peak from the Delta-driven surge (166,347 on September 1).

The CDC plans to update information about mask-wearing, including the different levels of protection that various masks -- such as cloth, surgical or N95 -- provide against the spread of Covid-19, CDC Director Dr. Rochelle Walensky said during a White House virtual briefing Wednesday.

Overall, it is important for people to wear any face mask that they have access to, "but Omicron has changed things a bit because it is so transmissible that we know that masks are even more important," Lori Tremmel Freeman, chief executive officer of the National Association of County and City Health Officials, told CNN on Wednesday.

"And if you have the chance, if you have the opportunity, if you have access to a better mask, then the recommendation would be to wear it," she said, adding that N95 and KN95 masks need to be fitted properly to provide the best protection possible.

Biden announced Thursday his administration would make "high-quality masks" available to Americans for free.

The President also announced his administration would purchase an additional 500 million Covid-19 tests -- on top of the 500 million tests he previously announced -- and said a website where Americans can go to get the free tests shipped to them will be rolled out next week.

Vaccines effective with adolescents, study shows

The rate of deaths in the US has remained lower than during last year's winter surge, which is often credited to around two-thirds of Americans eligible for vaccines being fully inoculated, according to the CDC.

The country has averaged 1,817 Covid-19 deaths a day over the past week, JHU data shows. The peak daily average was 3,402 one year ago on January 13, 2021.

However, the latest CDC ensemble forecast predicts a potential 62,000 new Covid-19 deaths over the next four weeks, meaning preemptive vaccinations are still needed.

The age group of Americans who are the least vaccinated remains those under the age of 18, and a new study of real-world hospital data between July and late October points to the effectiveness of vaccinations even for those who, by being younger, are generally at lower risk.

The findings, published Wednesday in the New England Journal of Medicine, show that the Pfizer/BioNTech coronavirus vaccine appears to be 94% effective against Covid-19 hospitalization among adolescents ages 12 to 18 in the US.

"Vaccination averted nearly all life-threatening Covid-19 illness in this age group," wrote the researchers from the CDC and a collection of hospitals and universities, who found that far more adolescents hospitalized with Covid-19 were unvaccinated compared with those who were hospitalized for other reasons.

Among the hospitalized adolescents with Covid-19, 4% were fully vaccinated, less than 1% were partially vaccinated, and 96% were unvaccinated. In comparison, of those who did not have Covid-19, 36% were fully vaccinated, 7% were partially vaccinated, and 57% were unvaccinated.