Delta, the highly contagious Covid-19 variant first identified in India, is becoming the dominant strain of the disease worldwide, the World Health Organization’s chief scientist said Friday.

That’s because of its “significantly increased transmissibility,” Dr. Soumya Swaminathan, the WHO’s chief scientist, said during a news conference at the agency’s Geneva headquarters. Studies suggest delta is around 60% more transmissible than alpha, the variant first identified in the U.K. that was more contagious than the original strain that emerged from Wuhan, China, in late 2019.

The situation globally “is so dynamic because of the variants that are circulating,” she added.

The variant has spread to more than 80 countries and it continues to mutate as it spreads across the globe, the WHO said Wednesday. It now makes up 10% of all new cases in the United States, up from 6% last week, according to the Centers for Disease Control and Prevention.

CDC Director Dr. Rochelle Walensky on Friday urged Americans to get vaccinated against Covid, saying she expects delta to become the dominant coronavirus variant in the United States.

“As worrisome as this delta strain is with regard to its hyper transmissibility, our vaccines work,” Walensky told the ABC program “Good Morning America.” If you get vaccinated, “you’ll be protected against this delta variant,” she added.

The United Kingdom recently saw the delta variant become the dominant strain there, surpassing alpha, which was first detected in the country last fall. The delta variant now makes up more than 60% of new cases in the U.K.

The WHO declared delta a “variant of concern” last month. A variant can be labeled as “of concern” if it has been shown to be more contagious, more deadly or more resistant to current vaccines and treatments, according to the health organization.

WHO officials said Wednesday there were reports that the delta variant also causes more severe symptoms, but that more research is needed to confirm those conclusions. Still, there are signs that the delta strain could provoke different symptoms than other variants.

Swaminathan said Friday that scientists still need more data on the variant, including its impact on the efficacy of Covid vaccines.

German company CureVac earlier this week cited variants as one of the reasons its Covid vaccine proved to be just 47% effective in a 40,000-person clinical trial.

An analysis from Public Health England released Monday found two doses of the Pfizer–BioNTech or the AstraZeneca Covid-19 vaccines are highly effective against hospitalization from the delta variant.

“How many are getting infected and of those how many are getting hospitalized and seriously ill?” Swaminathan said Friday. “This is something we’re watching very carefully.”

– CNBC’s Holly Ellyatt and Rich Mendez contributed to this report.