The U.S. Centers for Disease Control and Prevention now calls the Delta variant of the novel coronavirus, also known as B.1.617.2, a "variant of concern."

The variant of concern designation is given to strains of the virus that scientists believe are more transmissible or can cause more severe disease. Vaccines, treatments and tests that detect the virus may also be less effective against a variant of concern. Previously, the CDC had considered the Delta variant to be a variant of interest.

, which was first identified in India, shows increased transmissibility, potential reduction in neutralization by some monoclonal antibody treatments under emergency authorization and potential reduction in neutralization from sera after vaccination in lab tests.

The World Health Organization classified the Delta variant as a variant of concern on May 10.

A study on the Delta variant in Scotland published on Monday found that it was associated with about double the risk of hospitalization compared with the Alpha variant, B.1.1.7, that was first identified in the U.K..

At a White House COVID-19 briefing last week, National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci encouraged everyone to get vaccinated against COVID-19, noting that the Delta variant is now in circulation in the United States at a rate similar to the tipping point seen in the U.K.. There, the variant is now dominant.

"We cannot let that happen in the United States," Fauci said, adding it's "such a powerful argument" to get vaccinated. "Particularly, if you had your first dose, make sure you get that second dose. And for those who have been not vaccinated yet, please get vaccinated."

Dr. Peter Hotez, director of the Texas Children's Hospital Center for Vaccine Development, said on CNN's New Day Tuesday that he is "extremely worried" about the Delta variant, although two doses of Pfizer or Moderna's COVID-19 vaccines look like they function "really well" to protect against it.

Now is "crunch time," Hotez said, looking back to the surge of coronavirus cases across the South last year, when a "horrible" wave of cases emerged in July and August.

"I have to believe this, with this new Delta variant, the same thing is going to happen again with anyone who's either unvaccinated or only a single dose of vaccine," he said. "And so this is the time for everyone to get vaccinated, because even if you want to get yourself vaccinated tomorrow or your adolescent child tomorrow, it's still going to take five to six weeks to get both of those doses of vaccine and then another week after that."