In Arkansas, one of the states hit hardest by the delta variant, Gov. Asa Hutchinson has reversed course, asking the legislature to meet for a special session to lift a previously signed ban on mask mandates.

Hutchinson told "Good Morning America" Thursday that masks are not needed in schools if students are vaccinated but he's "particularly concerned" about kids under 12 who are too young to get the shot.

In those cases, he said, schools should have the option to enforce masks.

The governor added that he's pushing vaccine efforts with high school students and "making sure the faculty is vaccinated."

Moderna says its vaccine is 93% effective against symptomatic illness after six months (though this data collection ended before delta emerged in the U.S.).

Moderna said its booster candidates also demonstrate robust antibody responses to variants of concern, including delta.

Moderna President Dr. Stephen Hoge told "Good Morning America" Thursday that the Moderna vaccine offers protection from delta right after receiving it, but it's not known yet if that'll hold up through the winter. He called winter "the biggest test of that vaccine which is why we need to be vigilant and careful."

Hoge said he thinks booster shots will be needed and that the company is "preparing options," but he added that that'll be for public health officials, not the company, to decide.

He also noted that Moderna's goal is to be vaccinating kids under 12 by the end of the year. Moderna isn’t authorized for kids ages 12 to 17 yet, but Hoge hopes authorization could come “any day.”

Without control over the community spread of the delta variant, the virus has "ample opportunity to mutate," so "you may sooner or later get another variant," Dr. Anthony Fauci told "Good Morning America."

Fauci warned that it's possible a new "variant might be in some respects worse than the already very difficult variant we're dealing with now, which is a major reason why you want to completely suppress the circulation of the virus in the community."

There are still about 93 million eligible Americans who have not gotten vaccinated.

Fauci added, "People who say, 'I don't want to get vaccinated because it's me and I'll worry about me, I'm not having any impact on anybody else,' that's just not the case."

-ABC News’ Cheyenne Haslett

There are 5,042 new positive COVID-19 cases in Tokyo as of Thursday, according to the city's coronavirus information website.

Of those cases, 135 are severe and one has resulted in death.

It's a new record for Tokyo and a 178% increase since last Thursday, as the highly contagious delta variant spreads rapidly across the globe.

Mississippi is facing "a phenomenal increase in daily reported cases of COVID, and this is entirely attributable to the delta variant, which is sweeping over Mississippi like a tsunami," state health officer Dr. Thomas Dobbs said Thursday.

Dobbs said 97% of new cases are among the unvaccinated, and that 89% of hospitalizations and 85% of deaths are unvaccinated.

Dobbs pleaded with the public to get vaccinated, stressing that the unvaccinated population is driving the current surge, but that vulnerable, vaccinated people are suffering the fallout.

"There is going to be some collateral damage, unfortunately, even folks who've done everything they can to protect themselves," Dobbs said.

"The minority of folks who are vaccinated and hospitalized are overrepresented by the older and those with weaker immune system, so we're seeing a pretty dramatic spillover effect from the transmission in the community to more vulnerable parts of our population," he said.

-ABC News' Soorin Kim