(Bloomberg) -- The World Health Organization said the omicron variant of the coronavirus may change the course of the pandemic. It called on countries to vaccinate as fast as possible and keep measures in place to protect people from infection.

“We can prevent omicron becoming a global crisis,” WHO Director General Tedros Adhanom Ghebreyesus said at a briefing on Wednesday. “This virus is changing, but our collective resolve must not.”

The organization also said while there’s early evidence that omicron is milder than the delta strain, it’s too early to be definitive. 

“Certain features of omicron, including its global spread and large number of mutations, suggest it could have a major impact on the course of the pandemic,” Tedros said.

Asked about a new study from Pfizer Inc. and BioNTech SE on how their vaccine works against omicron, Kate O’Brien, director of immunization and vaccines, said the WHO is aware and will look at the findings.

“We are still in a delta pandemic, so vaccinate with existing vaccines continues to be the top priority,” she said.

Earlier on Wednesday, Pfizer and BioNTech said initial laboratory studies show a third dose of their vaccine may be needed to neutralize the omicron variant. The findings indicate that two doses “may not be sufficient” to protect against infection with the strain, they said, an analysis that will accelerate booster-shot drives around the world. 

The variant may be more transmissible than previous virus strains, evidence from South Africa indicates. Mike Ryan, executive director of the WHO’s emergencies program, said omicron appears to have higher transmissibility than delta, but “that does not mean the virus is unstoppable.”

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