The United States surpassed 500,000 deaths from COVID-19 on Monday, as key virus transmission indicators continued to fall and emerging variants of the virus caused new concerns.

"That's more lives lost to this virus than any other nation on Earth," US President Joe Biden said in a nationally televised address on Monday. "But as we acknowledge the scale of this mass death in America, we remember each person and the life they lived.

"We have been fighting this pandemic for so long. We have to resist becoming numb to the sorrow. We have to resist viewing each life as a statistic," he added.

The COVID-19 death toll matches the number of US deaths in World War II and the Korean and Vietnam wars combined. The US recorded an estimated 405,000 deaths in WWII, 58,000 in the Vietnam War and 36,000 in the Korean War.

As of Monday evening, the US COVID-19 death toll stood at 500,176, according to Johns Hopkins University. Earlier on Monday, Biden ordered US flags to be flown at half-staff for the remainder of the week in remembrance of the lives lost.

After his remarks, the president, first lady Jill Biden, Vice-President Kamala Harris and her husband, Doug Emhoff, led a national moment of silence at sundown from outside the White House. A military band played the hymn Amazing Grace, and 500 candles illuminated the steps of the South Portico of the White House.

The US reported its first COVID-19 case on Jan 19, 2020, in the state of Washington. One year later, on Jan 19, 2021, the last full day of Donald Trump's presidency, the country's coronavirus-related deaths reached 400,000.

"If you look back, historically, we've done worse than any other country, and we're a highly developed, rich country," Anthony Fauci, Biden's chief medical adviser, said on ABC's Good Morning America on Monday. "There were things back then, if you go back and think about what you might have done, the kind of disparate responses of different states, rather than having a unified approach, but you know, it's so tough to go back and try to do a metaphorical autopsy on how things went. It was just bad. It is bad now."

At the start of a three-day virtual national forum on COVID-19 on Monday, Rochelle Walensky, director of the US Centers for Disease Control and Prevention, said COVID-19 cases in the US have been declining for five weeks. She also said new hospital admissions are down and the daily number of reported deaths continues to fall, with the seven-day average slightly more than 1,900 deaths. That's a drop of 39 percent compared with the previous seven-day average.

"This is the lowest seven-day average since the beginning of December, but deaths remain nearly double the number reported during last summer's peak," she said.

Zhang Zuofeng, a professor of epidemiology and associate dean for research at the University of California, Los Angeles, School of Public Health, told Xinhua News Agency that "the vaccine rollout is the major reason for the decreasing of new cases, deaths and hospitalizations".

More than 75 million vaccine doses had been distributed across the US as of Monday, and more than 64 million doses have been administered, CDC data shows.

Variants' threat

Stanley Perlman, a professor of microbiology and immunology at the University of Iowa, said it is too early to know if this is a turning point because new, more contagious variants are circulating in the country.

The CDC said a total of 1,688 infections with coronavirus variants had been reported in the US as of Sunday, with 1,661 of them caused by the variant originally detected in the United Kingdom.

Long Xingchun, president of the Chengdu Institute of World Affairs, a nongovernmental think tank in Sichuan province, said the neglect of the pandemic situation and failure to make scientific decisions in curbing the virus on the part of the administration of former US president Donald Trump was the main factor contributing to the severity of the pandemic in the US. Moreover, its moves to politicize the virus on many occasions and attacks on the World Health Organization also damaged the world's joint efforts to contain the virus.

"The pandemic is a common enemy of mankind, and adopting a scientific approach toward the virus is the basis for the world to take joint efforts to effectively curb the pandemic," he said. Long added that it is a positive signal that Biden is attaching great importance to containing the virus and emphasizing global cooperation, which could benefit the world's fight against COVID-19.

Liu Yinmeng in Los Angeles and Xinhua contributed to this story.