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."

More than 65,000 public school staff and students in Los Angeles have tested positive for COVID-19 as the nation's second-largest school district returns to classrooms on Tuesday.

The Los Angeles Unified School District is requiring all employees and students to get tested for COVID-19 before returning for the Spring semester. Staff headed back to campuses on Monday, while the first day of classes for students was pushed back to Tuesday.

As of Monday evening, 424,230 employees and students have been tested and 65,630 were positive for the virus. The student positivity rate stands at 16.6% and the employee positivity rate stands at 14.9%, according to data released by the Los Angeles Unified School District.

"Our positivity rate remains lower than the overall county positivity rate as a result of our heightened safety measures and the continued partnership of families and employees," the school district said in a statement Monday evening.

Since the start of the pandemic, Los Angeles County has reported a total of more than 2 million cases of COVID-19, with a positivity rate of 21.4%, according to data released Monday evening by the Los Angeles County Department of Public Health.

The labs shouldering much of the nation's PCR COVID-19 testing are getting slammed with demand again during omicron's surge, and now they're grappling with a new challenge: their workforces are getting hit by the virus they've been tasked with tracking.

The American Clinical Laboratory Association, the national trade association representing some of the leading clinical labs responsible for COVID diagnostics, is warning that their members' workforce is strained as more workers call out sick.

"Labs are now facing a wave of new issues brought on by a fast-spreading variant that has not spared the laboratory care work force," an ACLA spokesperson told ABC News.

COVID-19 infections have increased laboratory staff sick leave -- a "significant factor in determining overall capacity" at an industry-wide level, the spokesperson said.

"We have been pressured to get our capacity where we believe it can be because of the labor problems we see," Quest Diagnostics CEO Steve Rusckowski said Wednesday at the JPM Healthcare Conference. "Some of this is just getting the labor to do our work, but secondly, is because of callouts because of the virus have been considerable over the last two weeks."

-ABC News' Sasha Pezenik