Some 20 million people across three Chinese cities are now under lockdown due to COVID-19 outbreaks.

Anyang, home to 5.5 million people, was the latest city to lock down its residents after discovering two cases of the fast-spreading omicron variant. Another 13 million people are under lock down in Xi'ian and 1.1 million in Yuzhou, with both cities still battling the highly contagious delta variant. Neither has reported any cases of omicron.

Meanwhile, restrictions have been imposed in the port city of Tianjin, about 80 miles southeast of Beijing, which is to host the 2022 Winter Olympics next month. The city's 14 million people are being tested for COVID-19 after two locally transmitted cases of omicron were detected over the weekend -- the first for mainland China.

-ABC News' Karson Yiu

Mexican President Andres Manuel Lopez Obrador has been reinfected with COVID-19.

Lopez Obrador, 68, who is fully vaccinated against COVID-19 and received a booster shot in December, revealed Monday evening that he has tested positive for the virus a second time.

"Although the symptoms are mild, I will remain in isolation and will only do office work and communicate virtually until I get through it," the president wrote on Twitter. "In the meantime, the interior secretary, Adan Augusto Lopez Hernandez, will take over for me at press conferences and other events."

The announcement came after two of the president's cabinet secretaries announced that they had tested positive for COVID-19 in recent days. Lopez Obrador attended a press conference earlier Monday without wearing a face mask.

The president, who has been accused of downplaying the highly contagious omicron variant as "a little COVID," contracted the virus for the first time and recovered in early 2021.

The American Red Cross said Tuesday that it is facing its worst blood shortage in over a decade.

"While some types of medical care can wait, others can't," Dr. Pampee Young, chief medical officer of the American Red Cross, said in a statement. "Hospitals are still seeing accident victims, cancer patients, those with blood disorders like sickle cell disease, and individuals who are seriously ill who all need blood transfusions to live even as Omicron cases surge across the country."

The American Red Cross, which supplies about 40% of the nation's blood, said it saw donor numbers fall as the delta variant of COVID-19 spread in August. The number of blood donors has fallen by about 10% since the beginning of the pandemic. That trend continued as omicron spread, according to the organization.

"We’re doing everything we can to increase blood donations to ensure every patient can receive medical treatments without delay, but we cannot do it without more donors," Young said. "We need the help of the American people."

The highly contagious omicron variant is expected to infect more than half of Europe's population within the next two months, the World Health Organization's top official in the region warned Tuesday.

Over 7 million newly confirmed COVID-19 cases were reported across Europe in the first week of 2022, more than doubling over a two-week period. So far, 50 countries in the region have detected omicron infections, according to Dr. Hans Kluge, the WHO's regional director for Europe.

Kluge said omicron, which was first identified in southern Africa in November, "represents a new west-to-east tidal wave" and is "quickly becoming the dominant [variant] in western Europe and is now spreading in the Balkans."

"As of Jan. 10, 26 countries report that over 1% of their population is catching COVID-19 each week," Kluge told reporters during a press conference Tuesday. "At this rate, the Institute for Health Metrics and Evaluation forecasts that more than 50% of the population in the region will be infected in the next six to eight weeks."