The Centers for Disease Control and Prevention estimates that more than 1 million people who have received the Moderna or Pfizer vaccine have gone back for an unauthorized third booster shot.

Florida is among the states reporting the highest number of people opting for a booster shot, followed by Ohio, California, Illinois and Tennessee.

The estimated 1.1 million, included in an internal CDC briefing document reviewed by ABC News, likely is an undercount because although it counts Moderna and Pfizer shot recipients it ignores people who may have received the one-shot Johnson & Johnson vaccine and got another shot.

It’s also unclear whether people who received a third shot did so under the direction of a doctor. The Food and Drug Administration hasn’t authorized a third shot to boost immunity, although there are reports of some physicians encouraging severely immunocompromised patients to do so.

Boosters for the immunocompromised may be recommended by the FDA within weeks.

-ABC News' Anne Flaherty, Eric Strauss

The proportion of hospitalized COVID-19 cases among vaccinated people has risen sharply since spring, coinciding with the sudden rise of the delta variant, according to the CDC's internal briefing slides reviewed by ABC News.

The finding doesn’t mean the vaccines aren’t working. People are still considerably less likely to wind up in the hospital or die if they are vaccinated, health officials have said.

The reality of more breakthrough cases also could be tied to the larger number of Americans getting vaccinated. While breakthrough cases remain relatively uncommon, the more people in the U.S. who are vaccinated increases the chances for breakthrough cases to be counted.

According to the CDC, vaccinated individuals who end up hospitalized tend to be elderly, medically vulnerable or live in long-term care facilities. They also are more likely to be asymptomatic and are hospitalized for reasons other than COVID-19.

On July 17, CDC Director Rochelle Walensky estimated that 97% of hospitalizations were occurring among people who weren’t immunized.

-ABC News' Anne Flaherty, Cheyenne Haslett, Eric Strauss

Dr. Anthony Fauci supports a mask mandate for teachers, he told MSNBC's "Morning Joe" Tuesday.

"We are in a critical situation here. We've had 600,000+ deaths and we are in a major surge now as we're going into the fall," Fauci said. "I know people must like to have their individual freedom and not be told to do something, but I think we're in such a serious situation now that, under certain circumstances, mandates should be done."

Fauci said it's only a matter of time until the FDA approves the vaccine and people shouldn't use that as a reason to delay getting the shot.

"There's no doubt that these vaccines are going to get fully approved," he said. "You should consider this as good as fully approved and get vaccinated."

-ABC News' Cheyenne Haslett

Only two intensive care unit beds were available for COVID-19 patients in hospitals in the Austin area Monday night, Austin-Travis County Health Authority Dr. Desmar Walkes said, according to ABC Austin affiliate KVUE.

There were 551 COVID-19 patients in area hospitals Monday, including 91 in the ICU, KVUE reported.

In the North Texas region, there are only two pediatric ICU beds available, Dallas County Judge Clay Jenkins said, according to ABC Dallas affiliate WFAA.

Children with COVID-19 used to make up 1% of patients hospitalized at Children's Hospital New Orleans. Now they account for about 20%, Dr. Mark Kline, physician-in-chief at Children's Hospital New Orleans, told "Good Morning America" Monday.

He said about half of the children hospitalized are under 2 years old. Most of the others are between 5 and 10 years old, so too young to be vaccinated.

"This is not your grandfather's COVID," Kline said. "This delta variant is an entirely new and unexpected challenge."

Dr. Peter Hotez, dean for the National School of Tropical Medicine at Baylor, warned on MSNBC that with "schools act[ing] as an accelerant, you should assume we're going to see pediatric intensive care units all across the South completely overwhelmed and even a possibility of small tent cities of sick adolescents and kids."

Hotez said parents need to know that "delta is something different" and "picking off young people like we've never seen."

"If your adolescent kid is unvaccinated, you should assume there's a high likelihood that that child is going to get COVID," he said, adding, "And we haven't even gotten to the 'long COVID' discussion around young people and what that means for their long-term cognitive health."