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

Pfizer's forthcoming COVID-19 treatment could be authorized for emergency use in the United States by the end of this year or early next year, pending the Food and Drug Administration's regulatory timeline, Pfizer CEO Albert Bourla told ABC News.

The pharmaceutical giant announced Tuesday that it has submitted promising new data to the FDA, including lab experiments showing its oral antiviral medicine, Paxlovid, will likely work against the omicron variant. And in updated clinical trial data, Pfizer found the treatment reduces the risk of being hospitalized or dying by 89% when taken within three days of being diagnosed with COVID-19, and 88% when taken within five days of being diagnosed among high-risk, unvaccinated patients.

"It is a game changer," Bourla told ABC News. "But at the same time, I want to emphasize that no one should use the existence of the pill as an excuse to avoid vaccination."

Bourla said the medicine could save thousands of lives.

"We did some calculations how many deaths or hospitalizations can be avoided based on the current U.S. mortality rates and the current U.S. hospitalization rates," he said. "On the back-of-the-envelope calculation, we estimate that 100,000 people if they take the pill, we will avoid 6,000 hospitalizations and 1,200 deaths approximately."

-ABC News' Sony Salzman