By Erika Edwards and Sara G. Miller - NBC News
The so-called delta variant (detected for the first time in India) already represents almost 10% of new coronavirus infections in the United States, the Centers for Disease Control and Prevention (CDC) reported on Tuesday.
The CDC also designated it as "concerning," an official consideration given when there is increased evidence of factors such as transmissibility, severity, or possible reduced effectiveness of vaccines or treatments.
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The change in classification "is based on growing evidence that the delta variant spreads more easily and causes more severe cases compared to other variants, including B.1.1.7 (alpha)," the CDC said.
The alpha variant was first detected in the United Kingdom and, in April, it became the dominant strain in the United States.
Data for the two-week period ending June 5 predict that the delta variant accounts for 9.9% of cases in the United States, the CDC said.
For the two-week period ending May 22, that figure was 2.7%.
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Still, there is evidence that vaccines in use in the United States are effective against this and other variants.
Two doses are required for the Moderna and Pfizer-BioNTech vaccines, while the Johnson & Johnson vaccine requires only one.
British health authorities reported that Monday that two doses of the Pfizer vaccine are highly effective (96%) in preventing hospitalization for this variant.
Still, Britain is experiencing a rapid delta spread, prompting Prime Minister Boris Johnson to delay reopening the country for a month, until July 19.
Because the delta variant can be transmitted from person to person more easily, experts say that those who have not received the vaccine remain especially vulnerable.
"This is the most dangerous phase of the looming pandemic for people who are not vaccinated," said Gregory Poland, director of the Vaccine Research Group at the Mayo Clinic in Rochester, Minnesota.
As of Tuesday, just over 54% of adults in the United States had been fully vaccinated and nearly 65% had received at least one dose.