The Centers for Disease Control and Prevention on Monday strengthened its recommendation on booster doses for adults. 

The previous recommendation was that all adults 50 and older should get a booster, and those 18 to 49 may want to get boosters. Now, the CDC says all adults should get a booster shot six months after their Pfizer or Moderna vaccines or two months after the Johnson & Johnson shot.

CDC director Rochelle Walensky said, "I strongly encourage the 47 million adults who are not yet vaccinated to get vaccinated as soon as possible and to vaccinate the children and teens in their families as well because strong immunity will likely prevent serious illness."

-ABC News' Eric M. Strauss

A 51-year-old man in Spain who traveled from South Africa on Nov. 28 has become Spain's first confirmed case of the omicron variant, according to the health ministry.

The man has mild symptoms and is under quarantine.

Sweden has identified its first omicron case, also a person who recently visited southern Africa, the Swedish Public Health Authority said.

A vaccinated 60-year-old woman has become the first confirmed omicron case in the Czech Republic, officials said. She visited Namibia in southwest Africa via South Africa and the United Arab Emirates. Eight people who traveled with the infected woman were contact traced and are now quarantined, officials said.

These countries also have confirmed omicron cases: Canada (2); the United Kingdom (11); Italy (1); Belgium (1); the Netherlands (13); Germany (3); Denmark (2); Portugal (13); Israel (1); Australia (5); Hong Kong (3); Botswana (19); and South Africa (exact number not clear).

Experts say the steady surge of infections is expected to only intensify in the weeks to come, after millions of Americans traveled and gathered over the Thanksgiving holiday.

Nationally, hospitalization numbers have ticked up to about 53,000, according to federal data. After nearly 10 weeks of steady declines, this marks the third consecutive week that the U.S. has seen an increase in hospitalizations.

Older populations are bearing the brunt of this latest surge, with Americans 65 and older accounting for more than 41% of current hospitalizations.

Minnesota and Michigan currently hold the nation's highest case rate, followed by Wisconsin, North Dakota, New Hampshire, Arizona, New York, Pennsylvania and Ohio, according to federal data.

-ABC News' Arielle Mitropoulos

ABC News' chief medical correspondent Dr. Jennifer Ashton said Monday that she "absolutely" believes omicron is already circulating in the U.S.

"When you hear the virus has been detected in so many countries, it should come as no surprise it's here," she said. "Viruses mutate for a living. As long as there are unvaccinated people in the world -- in South Africa, 6% vaccination rate -- this should not be a surprise."

Ashton said it will be critical for the U.S. to ramp up its genetic sequencing to effectively monitor the spread of new variants, including omicron.

Ashton, however, stressed that the U.S. in a much better place than one year ago.

"We're better at testing, we're better at surveillance, we're better at treating and we're better at preventing," Ashton said.

-ABC News' Arielle Mitropoulos

Three cases of the omicron variant have been detected in Maryland, all among Baltimore area residents, Gov. Larry Hogan said.

One of these people, who is vaccinated, recently traveled to South Africa. The second case is someone in their household who is not vaccinated, Hogan said.

The third case is unrelated; this person, who is vaccinated, has no recent travel, Hogan said.

No one required hospitalization, he said.