Molecular model of the SARS-CoV-2 coronavirus spike (S) protein (red) with highlighted mutation sites of variants B.1.531 and B.1.1.7. S proteins are located in the virus membrane, they bind to angiotensin converting enzyme 2 receptors (ACE2, blue) on host cell membranes and facilitate the entry of the virus into the cell.

Juan Gaertner | Science Photo Library | Getty Images

The highly contagious variant, first identified in the UK, is now the most common strain of Covid circulating in the US, the head of the Centers for Disease Control and Prevention said on Wednesday.

The variant known as B.1.1.7 is “now the most common line circulating in the United States”. CDC director Dr. Rachelle Walensky said during a briefing at the White House. “Testing remains an important strategy to quickly identify and isolate infectious individuals, including those with worrying variants,” said Walensky.

The UK identified B.1.1.7 last fall, which appears to be more deadly and spreads more easily than other strains. Since then, it has spread to other parts of the world, including the United States, which on Tuesday identified 16,275 cases in 52 jurisdictions, according to the CDC.

Florida has the highest number of confirmed cases of the new variant, according to a map from the CDC data, closely followed by Michigan, Wisconsin, California and Colorado. Public health officials say they are working as soon as possible to identify more cases.

Last week, Walensky said that the B.1.1.7 strain became the predominant strain in many regions of the United States, accounting for 26% of the Covid-19 cases circulating across the country. She urged the public to continue pandemic security measures such as hand washing, wearing masks and social distancing.

“This is a critical moment in our fight against the pandemic,” Walensky said last week. “We cannot afford to let go of our watch.”

The number of coronavirus cases is still well below its high in January, but slightly above the last low in late March. The seven-day average of daily new cases is 64,700, according to data from Johns Hopkins University, a level similar to that of the country during the summer surge.

Even if the pace of vaccinations accelerates, highly contagious variants will spread rapidly and could potentially delay the nation’s recovery from the pandemic.

Walensky said Wednesday the US needs to accelerate its vaccination efforts, which averaged 3.1 million shots a day on Monday. On Tuesday President Joe Biden announced that states should offer Covid-19 vaccine appointments to all adults in the United States by April 19, extending its original deadline by nearly two weeks.

“We must continue to vaccinate as many Americans as possible every day,” Walensky said, adding that new cases and deaths will decline.

– CNBC’s Nate Rattner contributed to this report.