“It is likely that Omicron will outpace the Delta variant where community transmission occurs,” the WHO said on Twitter, after updating its technical briefing on Omicron on Friday. The WHO said that as of Dec. 9, Omicron cases have been detected in 63 countries, up from 57 countries two days prior. The WHO said global risk levels regarding the variant remain “very high.”

While Omicron’s swift emergence and high rate of transmission have concerned experts, there are early indications that the variant may not be as deadly or overload hospitals with severe cases to the same degree as previous waves.

The WHO noted that preliminary findings indicate Omicron infections may be “less severe” than Delta, but the global health authority added that it is too early to make any firm conclusions about the strain’s severity. “More data are needed to understand the severity profile,” the WHO wrote.

The WHO said recent reports from the U.K., where the Omicron variant has spread widely despite the country’s high vaccination rate, are a particular concern. The U.K. has fully vaccinated 70% of its population, and 29% of U.K. citizens have also gotten booster shots. But experts warn that Omicron appears to be spreading faster in the U.K. than in South Africa, where the variant was first detected and where only 26% of people are fully vaccinated.

While Delta remains dominant in the U.K., Omicron cases have doubled every two to three days, prompting the U.K. government to raise its COVID alert level from three to four, one level below the government’s highest threat level.  

“No one should be in any doubt: There is a tidal wave of Omicron coming,” U.K. Prime Minister Boris Johnson said in a national address on Sunday.

“It is now clear that two doses of vaccine are simply not enough to give the level of protection we all need,” Johnson said. “But the good news is that our scientists are confident that with a third dose, a booster dose, we can all bring our level of protection back up.”

Preliminary research into the efficacy of vaccines against Omicron support Johnson’s claims. On Sunday, researchers in Hong Kong released a study showing the Pfizer-BioNTech vaccine is 32 times less effective in killing off the Omicron variant than Delta, matching a study in South Africa where researchers recently said that two shots of Pfizer’s vaccine may be 23% effective in stopping Omicron infections.

However, a U.K. study said on Friday that booster doses may raise protection levels from symptomatic cases to 75% against Omicron.

On Saturday, researchers at the Sheba Medical Center and Israel’s Ministry of Health likewise published initial findings indicating that booster shots provide additional protection against Omicron. The researchers found that Omicron was more evasive against vaccines than other variants including Delta, but said that people with a booster shot were less likely to get sick from Omicron than people who have had only two doses.

“If you’re past five months [from being vaccinated] and didn’t get a booster, go get it,” Dr. Gili Regev-Yochay, director of the epidemiology unit at Sheba Medical Center, said at a press conference on Saturday.