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.

Public school students in New Jersey will be required to wear masks when school starts up again, Democratic Gov. Phil Murphy announced Friday.

The rules, which will apply to K-12 students, are a departure for Murphy, who previously said school mask mandates would be left up to each district.

"I want as much as anybody else in our state to see our kids' smiles as they start their school years. But I do not want to see any of them getting sick needlessly or schools have to shut down again and go remote because of an outbreak," Murphy told reporters.

The governor stressed that the mask mandate is not permanent.

"As soon as conditions allow -- and please God sooner than later -- we will lift this requirement again," he said.

-ABC News' Will McDuffie