How the UK — where the Covid delta variant 'exploded' — is a blueprint for the U.S.

Medical staff member Mantra Nguyen installs a new oxygen mask for a patient in the Covid-19 intensive care unit (ICU) at the United Memorial Medical Center in Houston, Texas.

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Delta 'greatest threat' to U.S.

The first thing to note is how quickly the delta variant spread across the U.K.

In a relatively short amount of time, the strain supplanted the alpha variant to become dominant in the country (in mid-June delta was responsible for 90% of all infections, a government study showed) — and this happened despite the U.K.'s advanced vaccination rate.

Meanwhile, cases attributed to the delta strain now make up around 20% of newly diagnosed cases in the U.S. according to White House chief medical advisor Dr. Anthony Fauci.

Fauci warned last week that the delta variant is set to become the dominant Covid strain in the country in a matter of weeks, citing the U.K. as precedent. "It just exploded in the U.K. It went from a minor variant to now more than 90% of the isolates in the U.K.," Fauci said on NBC's "TODAY" show.

He said the variant has a doubling time of about two weeks. "So you would expect, just the doubling time, you know, in several weeks to a month or so it's going to be quite dominant, that's the sobering news," he added.

Read more: Fauci says delta accounts for 20% of new cases and will be dominant Covid variant in U.S. in weeks

Fauci had already warned that delta appears to be "following the same pattern" as alpha. "Similar to the situation in the U.K., the delta variant is currently the greatest threat in the U.S. to our attempt to eliminate Covid-19," he said.

In the U.K., infections attributed to delta have spread rapidly among young people and anyone older who has not yet been vaccinated. Similarly, in the U.S., there are concerns that delta could rapidly spread in parts of the South where vaccinations have stalled, NBC News reported Sunday.

The U.S. is not alone in its concern over the delta variant. In Australia, Sydney was put under a two week lockdown over the weekend amid a growing outbreak of the strain. The Australian government's Covid response team is set to meet Monday to discuss the spread of the virus and the possibility of more restrictions.

Vaccination rush

New outbreaks of infections largely blamed on the delta variant have prompted the U.K.'s government to speed up the last leg of its immunization program for people aged 18 and over.

It's hoped that stepping up vaccinations will help stop the wild spread of the strain. Analysis from Public Health England released June 21 showed that two doses of the Pfizer-BioNTech or the Oxford-AstraZeneca Covid-19 vaccines are highly effective against hospitalization from the delta variant.

To date, almost 60% of all U.K. adults have received two doses of the vaccine, while in the U.S., 56% of the population over 18 has been fully vaccinated. The U.K. has not yet authorized Covid shots for adolescents, unlike the U.S. which is giving vaccines to the over-12s.

Read more: Delta Covid variant has a new mutation called 'delta plus': Here's what you need to know

Perhaps wary of how infections have spread in the U.K., the U.S. wants to speed up its vaccinations too. It could take more time than the White House would like, however.

The Biden administration said last Tuesday that it likely won't hit its goal of 70% of American adults receiving one vaccine shot or more by the Fourth of July.

Read more: Covid boosters in the fall? As calls grow for third shots, here's what you need to know

White House Covid czar Jeff Zients said the administration had met its 70% target for people aged 30 and older and is on track to hit it for those aged 27 and older by July Fourth. Zients said U.S. officials were working with state and local leaders to reach younger people.

"We think it'll take a few extra weeks to get to 70% of all adults with at least one shot with the 18- to 26-year-olds factored in," he said.

-CNBC's Nate Rattner and Dawn Kopecki contributed reporting to this story.

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