The number of people who have died from COVID-19 in the United States surpassed 800,000 on Tuesday, according to real-time data compiled by Johns Hopkins University's Center for Systems Science and Engineering.

The figure is greater than the approximately 700,000 Americans who have died from AIDS-related illnesses over the last four decades, and it's higher than the total number of U.S. troops who have fallen in battle since 1900.

Since last December, when the first COVID-19 vaccines were being administered, an additional 500,000 people in the U.S. have died from the virus.

Of those, some 230,000 have died since April 2021, when U.S. President Joe Biden announced COVID-19 vaccines were widely available to every American over the age of 18.

-ABC News' Arielle Mitropoulos

The U.S. saw a sevenfold increase in the prevalence of the omicron COVID-19 variant over the last week, according to data from the U.S. Centers for Disease Control and Prevention.

Just over two weeks after it was first discovered in the country, the omicron variant is now estimated to account for nearly 3% of all new cases in the U.S., the latest data from the CDC shows.

Last week, omicron accounted for an estimated 0.4% of all new cases, according to the data.

-ABC News' Arielle Mitropoulos

Dr. Anthony Fauci told CNN Tuesday that omicron will "for sure" become the dominant strain in the U.S. given how rapidly it is spreading.

"Omicron is going to be a challenge because it spreads very rapidly," Fauci said.

Fauci reiterated that omicron so far appears to be less severe, adding, "Whether it is inherently less pathogenic as a virus or whether there is more protection in the community, we're just going to have to see when it comes in the United States."

-ABC News' Arielle Mitropoulos

Cornell University is moving into a "level red" alert after a "significant" number of suspected omicron cases were detected among student samples.

"While we must await confirmatory sequencing information to be sure that the source is Omicron, we are proceeding as if it is," university president Martha Pollack wrote in a letter to the community.

All final exams will be online beginning Tuesday, Pollack announced, and libraries and fitness centers are closed.

All undergraduate events are canceled, as is Saturday's recognition ceremony for December graduates, Pollack said.

Cornell has recorded more than 600 confirmed cases among students and staff in the last week alone, according to the university dashboard. While no infected students are seriously sick, Pollack said the university has "a role to play in reducing the spread."

-ABC News' Arielle Mitropoulos, Chris Donato

The omicron variant appears to have a "growth advantage" over the delta variant, the World Health Organization said in a technical brief released Sunday.

"It is spreading faster than the delta variant in South Africa where delta circulation was low, but also appears to spread more quickly than the delta variant in other countries where the incidence of delta is high, such as in the United Kingdom," the WHO said in the brief, which was dated Friday. "Whether omicron's observed rapid growth rate in countries with high levels of population immunity is related to immune evasion, intrinsic increased transmissibility, or a combination of both remains uncertain. However, given the current available data, it is likely that omicron will outpace the delta variant where community transmission occurs."

Meanwhile, preliminary findings from South Africa suggest omicron may cause less severe illness than delta, and all cases of omicron reported in Europe to date have been mild or asymptomatic. But the WHO said "it remains unclear to what extent omicron may be inherently less virulent" and that "more data are needed to understand the severity profile."

The WHO also noted that "there are limited available data, and no peer-reviewed evidence, on vaccine efficacy or effectiveness to date for omicron." However, preliminary evidence, and the considerably altered antigenic profile of the variant's spike protein, suggests a reduction in vaccine efficacy against infection and transmission associated with omicron.

"There is some preliminary evidence that the incidence of reinfection has increased in South Africa, which may be associated with humoral (antibody-mediated) immune evasion," the WHO said.

The diagnostic accuracy of routinely used polymerase chain reaction (PCR) test and antigen-based rapid diagnostic test (Ag-RDT) assays does not appear to be influenced by omicron. Therapeutic interventions for the management of severe or critical COVID-19 symptoms associated with omicron are also expected to remain effective, according to the WHO.

"However, monoclonal antibodies will need to be tested individually, for their antigen binding and virus neutralization and these studies should be prioritized," the WHO added.

-ABC News' Christine Theodorou