The World Health Organization (WHO) on Monday warned the global risk from the omicron coronavirus variant is "very high"—news that comes as CDC encouraged all eligible American adults to get booster shots of Covid-19 vaccines.

Omicron: The 3 biggest unanswered questions

WHO says risk from omicron is 'very high'

According to WHO, there are "considerable uncertainties" surrounding omicron, but early evidence shows the variant has mutations that could help it evade immune system response and increase its transmissibility.

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"Depending on these characteristics, there could be future surges of Covid-19, which could have severe consequences, depending on a number of factors, including where surges may take place," WHO said. "The overall global risk … is assessed as very high."

Theodora Hatziioannou, a virologist at Rockefeller University, said omicron's mutations suggest it may have evolved in the body of someone with a compromised immune system. The virus could have stayed in the person for months, evolving the ability to evade antibodies. "This virus has seen a lot of antibodies," Hatziioannou said.

Still, some experts say it's too early to know how effective Covid-19 vaccines will be against omicron. William Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health, said vaccines will most likely protect against the variant but more studies are needed to determine just how effective they'll be.

Omicron is "certainly enough to take seriously, but it's not apocalyptic," Hanage said. "It's not a magic virus."

CDC urges all American adults to get booster shots

Meanwhile, CDC Director Rochelle Walensky on Monday urged all American adults to get booster shots of Covid-19 vaccines, a change from the agency's previous recommendation that young, healthy adults "may" get a booster shot.

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"The recent emergence of the omicron variant further emphasizes the importance of vaccination, boosters, and prevention efforts needed to protect against Covid-19," Walensky said.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said that when a vaccinated person receives a booster shot, "you increase the level of neutralizing antibodies extraordinarily high—many fold higher than even the peak following the first two doses."

He added that "it is quite conceivable if not likely" that booster shots will protect against omicron.

However, Paul Offit, a professor of pediatrics at the Children's Hospital of Philadelphia, alongside Marion Gruber and Phillip Krause, two former FDA officials, authored an op-ed saying the United States should focus on reaching the unvaccinated rather than booster shots.

"To the contrary, the possible need for a booster shot targeting a potentially vaccine-resistant variant is a reason to hold off on a booster targeting the original variant," the authors wrote.

Celine Gounder, an epidemiologist and infectious disease specialist, said boosters should be part of the response against omicron, but focusing mostly on booster shots could leave much of the developing world unvaccinated, which creates an environment for more variants to emerge.

"You have one approach that has a likely short-term benefit versus another approach that has a very likely long-term benefit, and how do you weigh one versus the other?" Gounder said. (Keaten et al., Associated Press, 11/29; Zimmer, New York Times, 11/26; Rafford, Politico, 11/29; Shear/Stolberg, New York Times, 11/30; Lim/Gardner, Politico, 11/29; Abutaleb et al., Washington Post, 11/28; Loftus, Wall Street Journal, 11/29; Collis, Politico, 11/29; Walker, Wall Street Journal, 11/30; Schnell, The Hill, 11/29)