But since then, a more transmissible new strain of the coronavirus has taken hold, imperiling plans to fully reopen the country on June 21.

The potent new variant, known as Delta or B.1.617, emerged in India during that country’s recent coronavirus surge.

According to British Health Secretary Matt Hancock, it is around 40 percent more transmissible than the original strain, or wild type, that first appeared in 2019.

While several coronavirus variants have appeared since the start of the pandemic, epidemiologists worry that mutation could create a strain that evades vaccines.

The Delta variant accounts for only about 6 percent of cases in the United States, according to genomic sequencing studies.

“We cannot let that happen in the United States,” Fauci said, describing the scenario in the U.K.

The emergence of the Delta variant presents a new challenge because, as University of Edinburgh immunologist Eleanor Riley told the Financial Times, vaccines provide “somewhat less protection against infection with the Delta variant.” Even fully vaccinated people appear to develop fewer neutralizing antibodies against the Delta strain than for other variants.

Fauci also said in Tuesday’s briefing that the new variant may be “associated with increased disease severity” compared with the coronavirus wild type.

People who have had only their first dose of the two-dose vaccines from Pfizer and AstraZeneca appear to be especially vulnerable to the Delta variant, Fauci said on Tuesday.

(The AstraZeneca vaccine has been widely used in Europe but is not being administered in the United States; the Moderna vaccine, by contrast, is popular in the United States but not in the United Kingdom.) While both vaccines were about 50 percent effective against the original SARS-CoV-2 strain three weeks after the first dose, they were only 33 percent effective against the Delta strain.

Two weeks after the second dose, their effectiveness jumped to 88 percent for Pfizer and 60 percent for AstraZeneca, representing what were only slight decreases in effectiveness when compared with the original coronavirus strain.

The United States is in a similar position, having vaccinated 53 percent of its adult population fully, according to the Centers for Disease Control and Prevention; 64 percent are partially vaccinated.

Anthony Fauci urged Americans to get vaccinated to slow the spread of the Delta variant of the coronavirus.

“The Delta variant currently accounts for more than 6% of the sequenced cases in the United States.

This is a situation, the way it was in England,” Fauci said, noting how the Delta variant which originated in India, took over in the United Kingdom.

“We cannot let that happen in the United States,” Fauci said, urging all Americans to get vaccinated with both vaccine doses.