An increasing number of children are being infected with multisystem inflammatory syndrome in children (MIS-C), according to a statement Saturday relased by Los Angeles County Board of Supervisors Chair Hilda L. Solis, supervisor to the First District.

"It is heartbreaking that every day, more than 14,000 Los Angeles County residents are testing positive for COVID-19. It is even more painful to see an increasing number of children being infected by MISC-C. There is a misconception that COVID-19 only impacts those who are older or those with underlying health conditions. At this point in the crisis, we can say for certain that COVID-19 causes severe consequences in people of all ages, including children, and even without underlying health conditions. In Los Angeles County, the Latinx community is bearing the brunt of COVID-19 and MISC-C cases," the statement read.

There have been 51 total cases of MIS-C in children, including one child death. All 51 children with MIS-C were hospitalized and nearly 50% of the children were treated in the intensive care unit. Of the children with MIS-C, 31% were under the age of 5 years old, 38% were between the ages of 5 and 11 years old, and 31% were between the ages of 12 and 20 years old. Latinx children account for nearly 73% of the reported cases, according to the statement.

Experts say the inflammatory syndrome resembles toxic shock syndrome and Kawasaki disease, itself a rare inflammatory disease typically affecting children under the age of 5, and has the potential to be deadly.

-ABC News' Matthew Fuhrman, Olivia Rubin and Soo Rin Kim

Madrid health officials confirmed Saturday that four cases of the new variant of COVID-19 already detected the U.K, have been discovered in Spain, Reuters reported.

The new variant has also been detected in Sweden after a traveler from Britain fell ill on arrival and tested positive for it, the Swedish Health Agency said on Saturday, according to Reuters.

"The employee received the Moderna vaccine Thursday and as is our standard practice was being observed post vaccination by trained nurses. He felt he was developing an allergic reaction and was allowed to self-administer his personal epi-pen. He was taken to the Emergency Department, evaluated, treated, observed and discharged. He is doing well today," Boston Medical Center said in a statement to ABC News.

- Aaron Katersky

Despite health official warnings to stay home this holiday season, and while overall air travel is down nearly 60% from this time last year, the Transportation Security Administration's numbers have jumped, showing more than 7 million people hitting the skies in the past seven days, according to a "Good Morning America" report Saturday.

Over 7.8 million travelers were screened by the TSA since last Friday, Dec. 18 -- the busiest stretch of travel since the beginning of the pandemic.

The Centers for Disease Control and Prevention will now require a negative COVID-19 test for all air travelers coming to the United States from the United Kingdom in the wake of news about new strains of the virus being detected in the U.K.

Everyone arriving in the U.S. must have a negative PCR or antigen test from within 72 hours of departure, according to a statement from the CDC.

"The public health authorities in the United Kingdom recently announced the discovery of a new variant of SARS-CoV-2," the CDC wrote in its announcement. "Viruses constantly change through mutation, and preliminary analysis in the U.K. suggests that this new variant may be up to 70% more transmissible than previously circulating variants."

Passengers will have to provide written documentation of the negative test and airlines will be required to confirm the information, the CDC said. Airlines will have to refuse anyone without a negative test.

While the strain appears to be more transmissible, there's no evidence it is more deadly, or that the vaccines currently being rolled out won't be effective against it.

President Donald Trump, who is spending the Christmas holiday in Florida, will sign the executive order on Friday and it will go into effect on Monday.

Dozens of countries have largely banned travelers from the U.K., at least temporarily, including Canada, Mexico, Russia, China and India. The U.S. is one of a handful of countries that will require negative tests, including Greece and Spain.