WASHINGTON - Failure to adhere to mitigation procedures and quarantining has resulted in recent surge of COVID-19 cases across the United States, leading immunologists told Xinhua on Sunday.

Younger populations, especially those with co-morbidities, are being infected and hospitalized amid record daily spikes in confirmed cases, Stanley Perlman, a professor of microbiology and immunology at the University of Iowa, told Xinhua in an email interview.

More than 2,531,000 COVID-19 cases have been reported in the United States with the fatalities surpassing 125,600 as of Sunday afternoon, according to a tally by Johns Hopkins University.

Global confirmed COVID-19 cases topped over 10 million, with death toll reaching nearly 500,000, according to the tally.

The US Centers for Disease Control and Prevention (CDC) has reported 44,703 new confirmed cases and 508 new deaths on Sunday over previous day, according to its website. The updated count sets another new daily increase record.

Florida, Texas and Arizona are fast emerging as the country's latest epicenters after reporting record numbers of new infections for weeks in a row. Positivity rates and hospitalizations have also spiked.

Texas set a record for coronavirus-related hospitalizations for the 16th day in a row on Saturday, with 5,523 patients being treated. In Arizona, health officials also reported a record, with 2,577 current hospitalizations, according to The Washington Post.

Increasing gatherings and lack of social distancing amid reopenings nationwide have led to the deterioration of the pandemic in the country, said Zhang Zuofeng, a professor of epidemiology and associate dean for research with the school of public health at University of California, Los Angeles.

All 50 states have begun to reopen after the coronavirus thrust the country into lockdown starting in March. Mass gatherings are now on a rise, and increasing numbers of people, especially the young population, tend not to keep social distances, he told Xinhua.

"Meanwhile, some Americans remain highly resistant to wearing masks in public areas, thus they cannot well protect themselves and people around them. This is another important factor leading to the record-high daily increases," Zhang said.

US Secretary of Health and Human Services Alex Azar said on Sunday the "window is closing" for the country to curb the surge of coronavirus cases.

The number of people in the United States who have been infected with the coronavirus is likely to be 10 times as high as the confirmed cases, CDC Director Robert Redfield said earlier this week.

Whether the country will see increase of infections throughout the summer depends on adherence to mitigation and quarantining, Perlman told Xinhua.