By Susan Heavey and Carl O’Donnell

WASHINGTON (Reuters) – The highly contagious variant of COVID-19 first discovered in the United Kingdom has become the most common strain of the virus in the United States as cases continue to climb, a top U.S. health official said on Wednesday.

The strain, known as B.1.1.7, was identified in Britain last fall and has since been detected in 52 jurisdictions in the United States, U.S. Centers for Disease Control and Prevention Director Dr. Rochelle Walensky told reporters at a White House briefing.

U.S. public health officials have urged Americans to get vaccinated as soon as possible in part to prevent new variants of the novel coronavirus from spreading.

The United States has also detected cases of a variant first discovered in South Africa that is thought to be resistant to some COVID-19 vaccines and treatments. That strain has been found in 36 U.S. jurisdictions, according to federal data last updated on Tuesday.

The United States is administering about 3 million COVID-19 vaccine doses per day on average over the past week, up 8% over the previous seven-day average, Walensky said.

Vaccine supply has increased significantly in the United States in recent weeks as Johnson & Johnson has begun making millions of doses of its recently authorized shots. Pfizer/BioNTech and Moderna have also recently boosted their vaccine production capacity.

U.S. President Joe Biden has doubled his goal for shots administered in his first 100 days in office from 100 million to 200 million and urged states to begin giving shots to all adults by mid-April. (Open https://tmsnrt.rs/3tUM8ta in an external browser to see a vaccination graphic)

Still, daily U.S. cases of novel coronavirus are averaging 63,000 over the past seven days, up 2.3% from the previous seven-day average, Walensky said.

Walensky said that the CDC has identified a number of COVID-19 outbreaks tied to youth sporting events and that communities experiencing high case counts should avoid holding such events. Testing should also happen twice a week, she said.

White House COVID-19 adviser Andy Slavitt also told reporters that the U.S. government is expanding its community health center program, which it set up in recent weeks to help get vaccines into underserved communities.

(Open https://tmsnrt.rs/34pvUyi in an external browser to see a graphic of global COVID cases and deaths)

(Reporting by Susan Heavey, Jeff Mason and Carl O’Donnell; Editing by Lisa Shumaker)