The highly transmissible Delta variant is now the dominant strain of coronavirus in the United States, outperforming the alpha variant, according to Covid-19 modeling data released by the Centers for Disease Control and Prevention.

Delta, the variant first found in India and now found in at least 104 countries, accounted for 51.7% of new Covid cases in the US in the two weeks ending July 3, according to recently updated estimates by the CDC. The proportion of new cases caused by alpha, which was first found in the UK, was only 28.7% over the same period, according to the US agency.

In the past few weeks, US health officials have warned that Delta is on its way to becoming the dominant variant in the US, as its prevalence in the nation doubles about every two weeks. On June 22, a little over two weeks ago, White House chief medical officer Dr. Anthony Fauci that Delta accounted for about 20% of all new cases in the United States

The World Health Organization, which Delta has called the “fastest and fittest” variant to date, assumes that it will become the dominant form of the disease worldwide. According to a WHO report, Delta is about 55% more transmissible than the Alpha variant.

President Joe Biden again on Tuesday urged that all eligible Americans receive Covid vaccinations, stressing the importance of being protected against Delta.

Although the US is well on its way to meeting 160 million fully vaccinated people in the coming days, millions remain unvaccinated against Covid, “and because of this, their communities are at risk, their friends are at risk, the people they care about” said Biden are at risk. “

“This is an even bigger concern because of the Delta variant,” said the president.

There are still about 1,000 counties in the U.S. with a vaccination rate of less than 30%, said CDC Director Dr. Rochelle Walensky told reporters last week. The counties are mainly in the southeast and midwest, she said, and the agency is already seeing rising disease rates in these places due to the further spread of the delta variant.

Scientists and other health experts fear the variant will lead to a surge in new cases this fall, which will hit those who remain unvaccinated hardest, unless states can vaccinate more people.

“I think there are two Americas,” said Dr. Paul Offit, a pediatrician and vaccine advocate who served on advisory boards for both the CDC and the Food and Drug Administration. “There’s vaccinated America and unvaccinated America, and I think unvaccinated America will pay a price for that.”

The White House announced last week that it was deploying Covid-19 response teams across the country focused on fighting the variant. The teams, made up of officials from the CDC and other federal agencies, will work with communities at higher risk of outbreaks.