• CDC Director Dr. Rochelle Walensky said Monday that all U.S. adults "should" get a Covid vaccine booster. Previously the agency said all adults "may" get the shot.
• The strengthened recommendation comes as public health officials worry that the omicron variant could exacerbate an expected winter infection surge.
• The variant has not yet been detected in the U.S.

The Centers for Disease Control and Prevention on Monday strengthened its recommendation on Covid booster shots, telling all adults that they "should" get an additional dose amid growing concern about the newly identified omicron variant.

"The recent emergence of the Omicron variant (B.1.1.529) further emphasizes the importance of vaccination, boosters, and prevention efforts needed to protect against COVID-19," CDC Director Dr. Rochelle Walensky said in a statement Monday.

The CDC cleared booster doses for all adults earlier this month, saying everyone over 18 "may" get a shot if they wanted one. Only people 50 and over were told they "should" get the shots at the time. The agency is now giving its strongest recommendation for everyone 18 and older, saying they should get an additional shot six months after their initial Pfizer or Moderna series, or two months after their first Johnson & Johnson shot.

The World Health Organization, in a paper published Sunday, warned that global risk posed by omicron is "very high." The variant has more than 30 mutations on its spike protein alone, some of which are associated with higher transmission and reduced antibody protection. The WHO said these mutations could fuel future surges of infection with "severe consequences."

"Early data from South Africa suggest increased transmissibility of the Omicron variant, and scientists in the United States and around the world are urgently examining vaccine effectiveness related to this variant," Walensky said.

Though omicron is believed to be more infectious than the predominant delta variant, it is not yet clear how much the heavily mutated strain will affect the strength of currently available vaccines. Moderna and Pfizer said they will have more data on this in about two weeks. The companies maintain that they can adjust their vaccines relatively quickly to combat new variants.

"I don't think that the result will be the vaccines don't protect," Pfizer CEO Albert Bourla told CNBC's "Squawk Box" on Monday. "I think the result could be, which we don't know yet, the vaccines protect less."

Bourla said that Pfizer can develop a new vaccine within 100 days. Moderna CEO Stephane Bancel told CNBC on Monday that the company could deploy a higher-dosage booster shot against omicron quickly but a variant-specific vaccine could take months.

Though omicron has not yet been detected in the U.S., President Joe Biden on Monday said the country will face the variant sooner or later, and encouraged Americans to get booster shots for added protection. Walensky, in her Monday statement, urged people to get tested as well.

"I also want to encourage people to get a COVID-19 test if they are sick," Walensky said. "Increased testing will help us identify Omicron quickly."

The emergence of omicron comes as public health officials are already concerned about a winter Covid surge and declining vaccine strength.

A study published in the journal Science this month found that the Pfizer vaccine's efficacy at preventing infection declined from 86% to 43% from February to October. Moderna's vaccine dropped from 89% to 58%, and J&J's vaccine fell from 86% to 13% efficacy against infection in the same study.

However, Pfizer found that its booster dose provides 95% protection against symptomatic infection in a clinical trial of 10,000 people.