A patient is wheeled outside the emergency room of Maimonides Medical Center in Brooklyn, New York, the United States, on Dec. 3, 2020. (Photo by Michael Nagle/Xinhua)

It took only five days for the United States to add 1 million new COVID-19 infections, the highest growth rate since the outbreak of the pandemic. Experts are expecting the spread to worsen, anticipating new waves from December holiday gatherings on top of a potential surge from the Thanksgiving week.

The United States has set new world records for COVID-19 core metrics including hospitalizations, seven-day average case and death increases, as the total number of confirmed cases topped 15 million on Tuesday.

It took only five days for the country to add 1 million new infections, the highest growth rate since the outbreak of the pandemic in the country.

The nation recorded over 186,000 new daily cases and 1,500 deaths on Monday, bringing the seven-day average daily case increase to a record-high of 196,500 cases, according to data updated Tuesday by the U.S. Centers for Disease Control and Prevention.

The seven-day average daily deaths stood at 2,211 as of Monday, also the highest since April 25.

The country has averaged about 2,237 daily coronavirus deaths across a week, just below its highest-ever average of 2,241 set on April 24, CNN cited Johns Hopkins University data as showing.

Cases and hospitalizations have kept soaring in the country recently, as health care staff and facilities are under extreme pressure to take care of burgeoning numbers of patients.

Travelers wearing face masks line up for security checks at O'Hare International Airport in Chicago, the United States, on Nov. 25, 2020. (Photo by Joel Lerner/Xinhua)

There are 102,148 people currently hospitalized with COVID-19 in the United States, a new record since the onset of the pandemic, according to the COVID Tracking Project.

Hospitalizations are rising in 31 states from two weeks ago. The seven-day averages for deaths are at record levels in the Midwest, South, and West, according to the tracking project.

"A weekly view of the data shows the Thanksgiving data backlogs coming in for tests, cases, and deaths. We should still assume that many of these very high case and death numbers are because of the holiday effect," the project tweeted.

Though over 15 million coronavirus cases have been reported in the United States so far, health experts believe it is likely a vast undercount, partly because of limited testing capacity, especially early in the pandemic.

Experts are expecting the spread to worsen, anticipating new waves from December holiday gatherings on top of a potential surge from the Thanksgiving week.

Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, testifies at a House subcommittee hearing in Washington, D.C., the United States, on July 31, 2020. (Kevin Dietsch/Pool via Xinhua)

"We probably are just going to start seeing the brunt of what it means when you have people traveling and congregating in seemingly innocent settings," said Anthony Fauci, the country's top infectious disease specialist.

"The situation is that as we enter now from the Thanksgiving holiday season into the Christmas holiday season, it's going to be challenging," Fauci said in an interview with CNN.

Some states have imposed new restrictions in face of the worsening situation, to prevent their hospital systems from collapsing.

California issued a new stay-at-home order which goes into effect within 48 hours in regions with less than 15 percent intensive care availability. Tens of millions of people in the state's southern and San Joaquin Valley regions are now under the new stay-at-home order.

As of Friday, hospitals in Massachusetts will curtail elective procedures to free up staffing and beds for spiking coronavirus cases.

Pennsylvania is diverting COVID-19 patients to other facilities due to full emergency rooms, as the state faces an increasingly dire situation in hospital beds.

The United States has recorded more than 15.01 million cases with over 284,800 related deaths as of Tuesday afternoon, according to the real-time count kept by Johns Hopkins University.