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.

Dr. Stephen Hoge, president of American biotechnolgy company Moderna, said he's hopeful that vaccines can help the U.S. population achieve herd immunity against the novel coronavirus by mid-year.

"It really depends what you think herd immunity needs to be. But if you assume 50 to 70% of the population, then we're working hard ourselves and the other manufacturers to make sure that's a possibility really in the late spring, early summer," Hoge told ABC News chief anchor George Stephanopoulos in an interview Monday on "Good Morning America."

"It's ultimately going to depend upon the delivery of those vaccines, and so that's something that the states and the health care providers in this country are ultimately leading the way on as well as Americans deciding they want to receive that vaccine," he added. But we're optimistic that by the middle of the year, we'll be able to achieve those sorts of numbers."

Moderna is ramping up production of its COVID-19 vaccine and is working to clear any "bottlenecks" in the supply chain, according to Hoge.

"At this point, a lot of the logistical bottlenecks that we're running into are problems we can solve on our own," he noted. "We're in good shape."

Hoge, who was a resident physician in New York City, said data currently shows that existing vaccines are still effective against all emerging strains of the virus. But the variant first identified in South Africa "is of some concern because it looks like it could hide from the vaccine a little better than others," he said.

"So our approach in Moderna is going to be to develop a booster vaccine so that if the South African variant or any other variant becomes a concern, we'll be able to offer a way to identify that, prevent it from hiding from the vaccine," he said.