The proportion of Delta-variant coronavirus cases in the US doubled in two weeks, Dr. Anthony Fauci said on Tuesday.
Speaking at a press briefing, Fauci, the White House chief medical advisor, said the variant was the "greatest threat" to the US efforts to eliminate COVID-19.
The Delta variant now makes up 20.6% of the COVID-19 cases in the US. That is about double the rate seen on June 5, when the variant made up 9.9% of cases.
On May 22, 2.7% of cases were caused by the Delta variant, Fauci said.
He said the US seemed to be "following the same pattern" as the UK, where the variant quickly became dominant and now makes up 99% of cases.
Fauci, speaking during an interview on NBC's "Today" show on Wednesday morning, said that the Delta variant will become the dominant COVID-19 strain in the US within several weeks.
The nation's top infectious disease expert noted that the available COVID-19 vaccines in the country work "quite well against this problematic variant."
"It's the unvaccinated people that we're concerned about," Fauci said, adding, "If they are unvaccinated, they are at risk."
Data from the Centers for Disease Control and Prevention shows that just over 65 percent of US adults have received at least one vaccine dose.
When asked about children under the age of 12 who are not yet eligible to get jabbed, Fauci said that the "best way to protect" them against the coronavirus variants "is to bring the level of virus circulation in the community down."
"The best way to do that," Fauci explained, is for "adults who are eligible for vaccination to get vaccinated."
In an interview with "Good Morning America" on Friday, Dr. Rochelle Walensky, the director of the Centers for Disease Control and Prevention, said the variant would likely become dominant in the US in the coming months.
On Monday, the World Health Organization called the Delta variant the "fittest" strain of the coronavirus yet.
That's because — compared to the Alpha variant, which to date is still dominant in the US — the Delta variant is a lot more transmissible.
It also appears to cause more hospitalizations and seems more likely to break through the protection given by one dose of the Pfizer and AstraZeneca vaccines.
Two doses of those vaccines are still effective at preventing symptomatic disease.
It's unclear how much protection the Moderna and Johnson & Johnson vaccines give against the variant.
On Friday, President Joe Biden urged Americans to get fully vaccinated.
He said young adults are particularly vulnerable as they are less likely to be vaccinated and more likely to be socializing than older people.
