Just a little more than halfway into January, the U.S. has added a staggering 3.9 million new COVID-19 cases and more than 51,000 virus-related deaths.

The country's total COVID-19 death toll is now rapidly approaching 400,000 -- that's more than the number of Americans who died in World War I, Vietnam War and the Korean War combined and nearly as many Americans who died in World War II.

And it's by far higher than any other country's COVID-19 death toll.

"The numbers are quite dire," Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, told CNN Sunday night.

And each day, hundreds of thousands more infections continue to be reported. The U.S. has added more than 200,000 daily cases on all but three days this month. And the true number of infections may actually be several times higher than what's recorded, a recent study suggested.

Hotez estimates it's closer to a million new infections daily.

"This is a screaming level of transmission across the United States and people are scared, people are upset," Hotez added. "There is an enormous amount of work that's going to have to happen starting January 20."

Experts have warned that while vaccinations are underway, the country is still not out of the woods, with the incoming director of the Centers for Disease Control and Prevention warning Sunday there are "dark weeks ahead."

"By the middle of February, we expect half a million deaths in this country," Dr. Rochelle Walensky, told CBS' "Face the Nation."

"We still yet haven't seen the ramifications of what happened from the holiday travel, from holiday gathering, in terms of high rates of hospitalizations and the deaths thereafter," Walensky added.

HOSPITALS UNDER 'SEVERE STRESS'

More than 124,300 Americans are currently hospitalized with the virus nationwide, according to the COVID Tracking Project. To put that into perspective, that's more than double the country's COVID-19 hospitalization peak in spring, according to data from the project.

In their latest briefing, the University of Washington's Institute for Health Metrics and Evaluation projected hospitals in many states "will be under severe stress in the next four weeks."

That's already been the case in different parts across the country.

Los Angeles County -- the epicenter of California's COVID-19 crisis -- has battled a brutal surge of infections, hospitalizations and deaths for weeks now. County health officials reported Sunday more than 7,400 people remained hospitalized with the virus -- 23% of whom were in intensive care units.

Pennsylvania officials said late last week there were more than 4,900 people hospitalized with COVID-19 -- nearly double spring's peak.

In Georgia, one hospital told CNN affiliate WSB they were so full they had to treat patients in hallways and ambulances.

"We truly are in the darkest days," Dr. Deepak Aggarwal, with Northeast Georgia Medical Center, told the news station.

VARIANT COULD MEAN SPRING SURGE

Meanwhile, multiple U.S. states have now reported cases of a new COVID-19 variant first detected in the UK.

The number of cases of this variant are likely to "double every week," according to Dr. Scott Gottlieb, former Food and Drug Administration commissioner.

"In about five weeks, this is going to start to take over," Gottlieb said during an interview on CBS' "Face the Nation."

"The only backstop against this new variant is the fact that we will have a lot of infection by then so there'll be a lot of immunity in the population, and we will be vaccinating more people," he added. "But this really changes the equation and I think what we're looking at is a relentless strike from this virus, heading into the spring."

"We could have persistently high levels of infection in the spring until we finally get enough people vaccinated."

And while COVID-19 variants are not necessarily more deadly, they still can cause more deaths, Dr. Anthony Fauci told NBC Sunday.

"Even though on a one-to-one basis, it's not more virulent, meaning it doesn't make you more sick or more likely to die, just by numbers alone, the more cases you have, the more hospitalizations you're going to have, and the more hospitalizations you have, the more deaths you're going to have," Fauci said.

12 MILLION VACCINATIONS

Meanwhile, more than 12.2 million Americans have received their first dose of the COVID-19 vaccine and more than 31 million doses have been distributed across the country, according to the latest data from the CDC.

That's as the country is just days away from President-elect Joe Biden's inauguration. Biden has outlined a plan for vaccinations that includes opening up eligibility to more people, creating more vaccination sites and taking measures to increase vaccine supply and distribution.

"Our plan is as clear as it is bold: Get more people vaccinated for free. Create more places for them to get vaccinated. Mobilize more medical teams to get the shots in people's arms. Increase supply and get it out the door as soon as possible," Biden said last week.

And he's also said he is "convinced" his administration can meet their goal of administering 100 million vaccine shots in his first 100 days in office. Fauci has said he "strongly" believes that's doable.

"If we get about 70% to 85% of the people in the country vaccinated, we likely will get to that umbrella of herd immunity," Fauci told NBC last week. "We can start approaching some form of normality, but it's really going to be dependent on the uptake of vaccines."