Twenty-four states have seen an uptick of at least 10% in Covid-19 cases over the past week, Johns Hopkins University data shows, as health experts and the federal government keep pressing for more people to get vaccinated.

The rapid spread of the Delta variant of the coronavirus has only ratcheted up the pressure.

That variant, first identified in India, accounted for 51.7% of all new Covid-19 infections in the country over the two weeks that ended Saturday, the US Centers for Disease Control and Prevention has estimated.

"We should think about the Delta variant as the 2020 version of Covid-19 on steroids," Andy Slavitt, a former senior adviser to Joe Biden's Covid Response Team, told CNN on Wednesday. "It's twice as infectious. Fortunately, unlike 2020, we actually have a tool that stops the Delta variant in its tracks: It's called  vaccine."

For fully vaccinated people, the variant "presents very little threat to you, very unlikely that you're gonna get sick," he said
Full approval for vaccines from the US Food and Drug Administration could encourage more people to get vaccinated, Slavitt and other experts have said. The current vaccines distributed in the US are authorized for emergency use only. Full approval for the Pfizer vaccine could come as early as this month, Slavitt said Tuesday.

On Thursday, Pfizer said it was seeing waning immunity from its vaccine. The company is now picking up its efforts to develop a booster dose that will protect people from variants.

"Based on the totality of the data they have to date, Pfizer and BioNTech believe that a third dose may be beneficial within 6 to 12 months following the second dose to maintain the highest levels of protection," the company said in a statement emailed to CNN.

The company said booster doses of its vaccine, developed with BioNTech, produce levels of neutralizing antibodies that are five to 10 times higher than what's produced after two doses. And it added that it's also developing a new formulation for a booster dose that may protect people from new variants more thoroughly.
A Pfizer spokesperson later told CNN the company planned to file for emergency use authorization for a booster dose with the FDA in August.