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The US 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.

The CDC said the Delta variant, 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.

Covid-19 cases have been declining over the past few months in the United States, but there’s concern that could change as the pace of vaccinations slows and the Delta variant spreads. The CDC estimates it accounted for 9.9% of cases in the US as of June 5.

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 was in circulation in the United States at a rate similar to the tipping point seen in the UK, where the variant is now dominant.

“We cannot let that happen in the United States,” Fauci said, calling the UK’s experience “such powerful argument” to get vaccinated.

The variant is believed to be responsible for the most recent rise in cases in the UK and a study of cases 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 UK.

The UK announced Monday that the easing of coronavirus restrictions would be delayed another four weeks, until July 19, following a rise in cases and, in particular, the growing spread of the Delta variant.

Delta variant in the United States

As of Sunday, the Delta variant was responsible for about 10.3% of US Covid-19 cases, according to Dr. Eric Topol, the founder and director of the Scripps Research Translational Institute, whose has been tracking variants throughout the pandemic.

The may not seem like a lot, but the speed with which it’s spreading is a concern.

“It doubles every seven to 10 days, which means when it gets to three weeks from now, this variant will be dominant,” Topol said. “That means we have two to three weeks to just go flat out with vaccination to stop this trend.”

Vaccinations generally seem to keep variants in check. The Alpha strain, for instance, is the dominant strain in the United States and has been since about late April. But with exceptions such as Michigan, it didn’t cause surges in cases in most parts of the country.

Topol said that the US was able to “ante up” and go “full tilt on vaccination” when the variant arrived in the United States.

With the Delta variant, Topol isn’t as optimistic.

“This is the most troubling variant by far, because it’s another 60% more contagious than the Alpha, so it’s a super spreader strain,” Topol said. But the vaccination rate is stalling.

While 43.9% in the US is fully vaccinated, according to the CDC, the rate at which people are getting vaccinated has been slowing down.

In Mississippi, nearly 29% of population is fully vaccinated. In Alabama it’s less than 31%. In Arkansas, less than 33%. In Louisiana, Georgia, and Wyoming it’s less than 34%, according to CDC data.

Vaccines and the Delta variant

The good news is that people who are fully vaccinated seem to have solid protection against the Delta variant.

A study published in the Lancet found a single dose of Covid-19 vaccine wasn’t enough, but after the second dose, the Pfizer-BioNTech provided 79% protection from the Delta variant. That compares with 92% protection against the Alpha variant.

Another analysis from England’s public health agency found two doses of the Pfizer vaccine seemed 96% effective against hospitalization.

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.

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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.”

CNN’s Naomi Thomas contributed to this report.