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

At least 10 people have tested positive for COVID-19 on a Cruise Norwegian ship disembarking in New Orleans this weekend, according to the Louisiana Department of Health.

Those infected include both passengers and crew members, officials said.

The Norwegian Breakaway departed from New Orleans on Nov. 28 and stopped in Belize, Honduras and Mexico on its voyage. There are more than 3,200 individuals on board, the health department said.

The company required on-site testing in addition to proof of vaccination before boarding the vessel, according to the Port of New Orleans.

The health department said the cruise line has been adhering to appropriate quarantine and isolation protocols. In addition, everyone aboard the ship will be tested for COVID prior to disembarking.

Those who test positive for COVID-19 on the ship will either travel by personal vehicle directly to their private residence or self-isolate according to current Centers fo Disease Control and Prevention guidelines in accommodations provided by Norwegian Cruise Lines.

"The health and safety of our guests, crew and the communities we visit is our highest priority, and we will continue to take all appropriate actions to ensure their wellbeing and protect public health," a Norwegian Cruise Lines spokesperson told ABC News.

"We have identified a handful of COVID-19 cases among guests and crew onboard Norwegian Breakaway, which is scheduled to disembark in New Orleans, LA on December 5. All of the identified cases onboard are asymptomatic. In addition to requiring that 100% of guests and crew are fully vaccinated, per the Company's comprehensive health and safety protocols, we have implemented quarantine, isolation and contact tracing procedures for identified cases."

The spokesperson said the company is testing all individuals prior to disembarkation, and providing post-exposure and quarantine public health guidance by the CDC.

"Any guests who have tested positive for COVID-19 will travel by personal vehicle to their personal residence or self-isolate in accommodations provided by the Company according to CDC guidelines," the spokesperson said. "We take this matter extremely seriously and will continue to work closely with the CDC, the office of Governor John Bel Edwards, the Louisiana Department of Health as well as the city and port of New Orleans. At this time, there have been no changes to scheduled future sailings on Norwegian Breakaway. We will provide additional updates to impacted guests as appropriate."