By ARIELLE MITROPOULOS, ABC News

(WASHINGTON) — The U.S. reached yet another grim milestone Tuesday as the confirmed coronavirus death toll topped 400,000, just one day before President-elect Joe Biden is set to take the oath of office.

The number of lives lost could fill Madison Square Garden in New York City nearly 20 times over and is roughly equivalent to the population of Tampa, Florida, or Tulsa, Oklahoma.

The 400,000 lives lost is more deaths than the number of U.S. soldiers that died in battle during World War I, World War II and the Vietnam War combined, an analysis of data compiled by the Department of Veterans Affairs shows.

The milestone comes just over a month after the country surpassed 300,000 confirmed COVID-19 deaths on Dec. 14, and only 17 days since the U.S. hit 350,000 reported deaths on Jan. 2, according to data collected by Johns Hopkins University.

“That the U.S. reached the grotesque milestone of 400,000 [COVID-19] deaths should stop us in our tracks. We should demand from our leaders an explanation of why they’ve allowed, in the last month alone, 100,000 Americans to die without taking action to stop this horrific loss of life,” Jennifer Nuzzo, a public health researcher at Johns Hopkins, told ABC News.

Globally, the virus has claimed more than 2 million lives. The U.S. accounts for approximately 19.5% of the world’s COVID-19 related deaths — about 188,000 reported deaths ahead of the second hardest-hit nation, Brazil. The United States makes up just over 4% of the global population.

Experts had been warning for months that if Americans did not take the necessary safety precautions to protect themselves and others over the winter — and in particular, over the holidays — many people would suffer the consequences.

During a White House Task Force press briefing March 29, President Donald Trump said that if the U.S. could keep the death toll between 100,000 to 200,000 lives lost, it would mean that the administration had “done a very good job.”

However, by Labor Day, there were already 189,000 Americans dead, and in October, Dr. Anthony Fauci, the nation’s top disease expert, warned that the number of fatalities could dramatically increase.

“The models tell us if we don’t do what we need to in the fall and winter, we could have 300,000 to 400,000 COVID-19 deaths,” Fauci said during a virtual event with the American University in Washington, D.C.

The totals have far eclipsed the task force’s early estimates, and now, the incoming Biden administration is warning that the “dark winter” is not over yet. Dr. Rochelle Walensky, who Biden nominated to serve as the next director of the Centers for Disease Control and Prevention, told CBS’ “Face the Nation” on Sunday that by the middle of February, “we expect half a million deaths in this country” from COVID-19.

“We still yet haven’t seen the ramifications of what happened from the holiday travel, from holiday gatherings, in terms of high rates of hospitalizations and the deaths thereafter,” Walensky added.

The U.S. currently averages just under 3,300 new coronavirus related deaths a day, and the beginning of the month, there have been more than 53,000 COVID-19 related deaths recorded — that is approximately 1 American death reported every 30 seconds.

“We can’t let ourselves get numb to this moral and leadership failing. We must double down on efforts to stop the virus from spreading,” Nuzzo said.

The milestone also comes just days shy of the one-year anniversary of the country’s first confirmed COVID-19 case, which was reported on Jan. 21, 2020, according to the CDC.

U.S. case totals now top 24 million, but the national seven-day average of daily cases is currently trending down from the record high set earlier last week — averaging approximately 207,000 new confirmed cases a day.

This is a drop from the record high reported last Monday, though the country has not seen a day with fewer than 100,000 reported cases in 11 weeks.

Hospitalizations have also dropped nationally, with just under 124,000 patients currently hospitalized across the country, down from 132,000, earlier this week.

The staggering COVID-19 numbers also follows news of a new CDC report, which warned that the U.K. variant could worsen the pandemic in the U.S., without “universal and increased compliance” with mask-wearing and social distancing. The new variant, which is believed to be more contagious than previous strains, has now been discovered in at least 20 states, according to an ABC News count.

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.”

And as the death toll continues to rise, state officials continue to clamor to vaccinate their residents, in a desperate effort to slow the death rate.

“Every vaccine dose sitting in a warehouse rather than going into an arm could mean one more death that could have been avoided,” Health and Human Services Secretary Alex Azar concluded last week.

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