Dr. Anthony Fauci, chief medical adviser to U.S. President Joe Biden, said new, more contagious variants of the novel coronavirus are the top concern in the country right now and that the next six weeks will be critical.

"We're going to be doing … a full-court press on non-pharmacologic interventions [like masks] as well as getting as much vaccine out as we possibly can," Fauci, who is also the director of the National Institute of Allergy and Infectious Diseases, said at the International Aids Society conference on Tuesday.

"It's a very stressful situation ... when you have that much virus circulating, you're going to get a lot of mutations, no doubt about it," he added. "It's almost a race of trying to suppress the level of replication before we get so many accumulation of both the South African and other mutants as well as mutants of our own."

ABC News’ Cheyenne Haslett contributed to this report.

Maryland has identified two more confirmed cases of the new, more contagious B1351 variant of the novel coronavirus, which was first detected in South Africa, Gov. Larry Hogan said Tuesday.

The two patients are Montgomery County residents who recently traveled abroad, according to Hogan.

"Contact tracing is underway, and close contacts are isolating," the governor wrote on Twitter.

Maryland’s first case of the South Africa variant, in the Baltimore region, was announced on Saturday.

"The B.1.351 variant has not been shown to cause more severe illness or increased risk of death, though it is believed to be more transmissible," Hogan tweeted. "Initial evidence suggests that vaccines are still likely to be protective against the variant."

Capt. Sir Thomas Moore, the 100-year-old British World War II veteran who garnered global attention for his fundraising efforts amid the pandemic, has died after contracting COVID-19.

His death was announced Tuesday on social media accounts run by his family.

Moore was being treated for pneumonia over the past few weeks. After testing positive for COVID-19 last week, the centenarian was hospitalized on Sunday because "he needed additional help with his breathing," his daughter said in a statement posted on Twitter.

Last spring, Moore raised over 37 million pounds ($50.7 million) for the United Kingdom's National Health Service by walking laps in his garden in England amid a nationwide lockdown. Queen Elizabeth II honored Moore's charity and service to the country with a promotion to honorary colonel, a social flyover and knighthood.

A Buckingham Palace spokesperson told ABC News the queen "is sending a private message of condolence" to Moore's family and that she "very much enjoyed" meeting them last year.

"Her thoughts, and those of the Royal Family, are with them, recognising the inspiration he provided for the whole nation and others across the world," the spokesperson said.

ABC News' Zoe Magee contributed to this report.

Andrew Yang, who ran for president of the United States in 2020 and is currently running to be mayor of New York City, has tested positive for COVID-19.

"After testing negative as recently as this weekend, today I took a COVID rapid test and received a positive result," Yang said in a statement through his campaign Tuesday.

Yang said he has mild symptoms and that, as he self-quarantines at home, he'll "continue to attend as many virtual events as possible."

"Our team has begun the contact tracing process to notify anyone who may have been in close contact with me," he added.

There were 111,896 new cases of COVID-19 confirmed in the United States on Sunday, according to a real-time count kept by Johns Hopkins University.

Sunday's case count is the lowest the country has recorded since Dec. 25 and is also far less than the all-time high of 300,282 newly confirmed infections on Jan. 2, Johns Hopkins data shows.

An additional 1,794 fatalities from COVID-19 were registered nationwide on Sunday, down from a peak of 4,466 new deaths on Jan. 12, according to Johns Hopkins data.

COVID-19 data may be skewed due to possible lags in reporting over the Martin Luther King Jr. Day holiday weekend.

A total of 26,187,424 people in the U.S. have been diagnosed with COVID-19 since the pandemic began, and at least 441,331 have died, according to Johns Hopkins data. The cases include people from all 50 U.S. states, Washington, D.C., and other U.S. territories as well as repatriated citizens.

Much of the country was under lockdown by the end of March as the first wave of pandemic hit. By May 20, all U.S. states had begun lifting stay-at-home orders and other restrictions put in place to curb the spread of the novel coronavirus. The day-to-day increase in the country's cases then hovered around 20,000 for a couple of weeks before shooting back up over the summer.

The numbers lingered around 40,000 to 50,000 from mid-August through early October before surging again to record levels, crossing 100,000 for the first time on Nov. 4, then reaching 200,000 on Nov. 27 before topping 300,000 on Jan. 2.

So far, the U.S. Food and Drug Administration has authorized two COVID-19 vaccines for emergency use -- one developed by U.S. pharmaceutical giant Pfizer and its German partner BioNTech, and another developed by American biotechnology company Moderna and the National Institute of Allergy and Infectious Diseases. More than 32 million vaccine doses have been administered nationwide, according to the latest data from the Centers for Disease Control and Prevention.