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, an analysis that will accelerate booster-shot drives around the world.

The WHO has been pushing for countries to hold off on boosters to make more vaccines available to poorer countries where inoculation rates are low. But governments may be less likely to do that if evidence from tests continues to show that third shots are needed to protect against omicron.

The variant has now spread to 57 countries, and it appears to be more transmissible than previous virus strains. Mike Ryan, executive director of the WHO’s emergencies program, said while omicron appears to have higher transmissibility than delta, “that does not mean the virus is unstoppable.”