President Joe Biden on Friday doubled down on his administration's plea to Americans to get vaccinated against Covid-19 as quickly as possible, warning the highly transmissible delta variant appears to be "particularly dangerous" for young people.

Biden said the best way young people can protect themselves is to get fully vaccinated.

The World Health Organization's chief scientist said earlier Friday that delta is becoming the dominant strain of the disease worldwide.

Studies suggest delta is around 60% more transmissible than alpha, the variant first identified in the United Kingdom that was more contagious than the original strain that emerged from Wuhan, China, in late 2019.

Rochelle Walensky also said Friday that she expects delta to become the dominant variant in the United States and urged people to get vaccinated.

"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.

Health experts say the delta strain is particularly worrisome for young people, many of whom have yet to get vaccinated.

Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia, said the delta variant has essentially replaced alpha, the variant that swept across Europe and later the U.S.