COVID-19 updates: Elizabeth Warren tests positive

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

South African President Cyril Ramaphosa is receiving treatment for "mild COVID-19 symptoms" after testing positive for the virus on Sunday, his office said in a statement.

Ramaphosa, 69, began feeling unwell earlier Sunday after leaving a state memorial service in Cape Town in honor of Frederik Willem de Klerk, South Africa's last apartheid president and a Nobel laureate, who died last month. Ramaphosa, who is fully vaccinated against COVID-19, was experiencing "mild" symptoms and a test confirmed he was infected, according to his office. The statement didn't say whether he has the omicron variant, which was discovered by scientists in southern Africa last month and is spreading rapidly.

Ramaphosa is self-isolating in Cape Town and is being monitored by the South African Military Health Service. He has delegated all his responsibilities to Deputy President David Mabuza for the next week, his office said.

Last week, Ramaphosa traveled with a delegation to four West African nations. He and the members of the South African delegation were all tested for COVID-19 in each of the countries during their trip. They returned to South Africa on Dec. 8, after testing negative in Senegal. Ramaphosa tested negative again upon arriving in Johannesburg that day, according to his office.

The statement advised people who had contact with the South African president on Sunday to watch for symptoms or to get tested for COVID-19.

"President Ramaphosa says his own infection serves as a caution to all people in the country to be vaccinated and remain vigilant against exposure," his office said in the statement. "Vaccination remains the best protection against severe illness and hospitalization."

-ABC News' Christine Theodorou