(NEW YORK) — The number of Americans who have died from COVID-19 has now eclipsed 600,000, 15 months since the onset of the pandemic.

The milestone is a sobering reminder that hundreds of Americans are still dying each day even as the nation begins to enter its “new normal.”

It was just over a year ago when the country recorded 100,000 confirmed virus-related deaths.

“A year ago, we were already stunned by the sheer loss of life at the 100,000 milestone and now we recognize that the impact was far greater than we could have imagined,” said Dr. John Brownstein, chief innovation officer at Boston Children’s Hospital and an ABC News contributor.

The Proceedings of the National Academy of Sciences estimates that 5.4 million Americans have lost a loved one because of COVID-19.

The U.S. COVID death toll is now more than 200 times higher than the number of lives lost during the U.S. attacks on Sept. 11. It is now approaching the total number of American deaths that were recorded during the 1918 influenza pandemic.

For context: 600,000 people could fill Yankee Stadium — eleven times — or Boston’s Fenway Park 16 times over.

“It is still very real. It is still something that is very serious and should be taken very seriously,” Shamayne Cruz, a respiratory therapist at UCHealth Memorial Hospital Central in Colorado Springs, told ABC News.

Globally, the virus has claimed more than 3.8 million lives. The U.S., which makes up just over 4% of the global population, accounts for approximately 16% of the world’s COVID-19 related deaths. The U.S. has the highest death toll of any country in the world.

“This pandemic is really beyond anything that we’ve ever experienced and yet another milestone highlights the fact that we are still not yet out of the woods,” Brownstein said.

The U.S. surpassed 500,000 confirmed COVID-19 deaths on Feb. 22, according to data collected by Johns Hopkins University.

Since the country’s viral peak in January, the average number of daily cases and deaths has plummeted by over 90%. Hospitalization levels too have fallen dramatically in recent months, with admission numbers down by more than 60% since mid-April.

The U.S. is currently averaging just under 350 new coronavirus-related deaths a day, with the nation reporting around 2,450 deaths a week, significantly lower than the 23,000 deaths reported over a seven-day period in January.

And with approximately 52.5% of the total U.S. population now vaccinated with at least one dose, states are quickly moving to drop coronavirus restrictions and face covering requirements for residents, signaling a modified return to pre-pandemic times.

Health officials, however, remain concerned about rapidly mutating variants and unvaccinated Americans.

President Joe Biden and other health officials are now urging young Americans to get vaccinated, given the potential threat of the Delta variant, which is spreading rapidly among younger populations in the United Kingdom.

According to data from the U.S. Department of Health and Human Services, the prevalence of the Delta variant, first identified in India, is increasing across the country. It now accounts for approximately 10% of new virus cases in the U.S.

Experts have warned that the Delta variant is not only more transmissible, but it can also cause more severe illness. It is particularly dangerous to those who are unvaccinated or partially vaccinated. Current evidence so far suggests that mRNA COVID-19 vaccines are effective against the Delta variant.

In an interview with ABC News’ David Muir on Wednesday, Dr. Ashish Jha, dean of the Brown University School of Public Health, described the Delta variant as “the most contagious variant we’ve seen so far” and warned that it has “really caused devastation in country after country.”

Front-line workers are also now reporting that a larger share of young, unvaccinated populations are receiving care. Nationwide, the 18-49 age groups continue to account for the largest number of patients hospitalized.

“Right now, what we find is we still do have people passing away from COVID, and those steps are challenging because from a nursing standpoint we feel like they could have been prevented with the vaccine,” Megan Bowes, a pulmonary and respiratory care unit manager at Health First’s Holmes Regional Medical Center in Florida, told ABC News. “It makes it sometimes even harder to wrap our brains around those people that are passing away still.”

The key to preventing more of these tragic milestones will ultimately be getting more people — globally and domestically — vaccinated, experts say.

“The vaccine is our ticket out of future tragic milestones,” Brownstein said.

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